Is two hours enough time in London between Norwegian Flight and Easy Jet to Milan

Nah, don't do it.

It sounds like you are planning to buy two separate tickets and not considering that the Norwegian flight from the USA could be delayed 2 hours causing you to miss your Easy Jet flight. Even if your first flight is on time If it is two separate tickets that is a still a recipe for disaster. You will find a long line at the Border Force (customs) at London Gatwick and then a long line through security and you will miss your flight to Milan.

However Norwegian has a partnership with Easy Jet to sell flights as one ticket; please clarify if you bought one ticket where Norwegian connects to Easy Jet and transfers your bag to the flight to Milan

On Kayak, it looks like the combined ticket is offered by Kiwi and Gatwick connects. Will I be able to buy the EasyJet ticket on the Norweigan site?

after I checked you will not be able to buy this on Norwegian's site; if you choose the aggregator (kayak) just make sure you are one ticket to Milan because after further review I see that there are 5 direct flights operated a day by Easy Jet to Milan MXP and Milan (linate) from Gatwick; the ailrine will put you on one of those if your flight form the USA is delayed

I have never used it but it looks like Gatwick Connects offers protected connections.

https://www.gatwickairport.com/at-the-airport/flying-in/gatwickconnects/

Probably best to check the fine print.

Also be aware that Norwegian and Easyjet operate from two different terminals at LGW.

Last spring I flew Norwegian into LGW from Norway and the Easyjet out to Jersey. Fortunately, I had Registered Traveller that got me through immigration in minutes. The regular line snaked through the arrival hall and up the ramp. The wait from the back was at least an hour.

https://www.gatwickairport.com/at-the-airport/flying-in/flight-connections/

Be sure to read thoroughly as easyjet is a special case.

If I were doing this, I would give myself more time.

Note easyjet ends boarding 30 minutes prior to departure--- https://www.easyjet.com/en/help/boarding-and-flying/boarding-and-on-board-services

Thus you have only 90 minutes, IF the inbound flight is on time.

Choose wisely and good luck

If you have a later EasyJet option, take it, as you will feel more relaxed rather than trying to rush through a strange airport with jet lag, changing terminals etc.

Although buying a through ticket means the onus is on the provider to get you on a later plane if necessary, the next plane maybe full, so you will get to your destination, but it could even be the following day if flights are full.

You don't say which London airport, but other posters have assumed Gatwick. Is that correct?

Easyjet does not interconnect with anyone, not even with its own flights. It is a strict "point-to-point" airline. Easyjet also has a strict cut-off for check-in and for getting to the gate. Check-in desks close exactly 40 minutes prior to the scheduled departure time, 1 minute late and they won't let you on.
Gates close promptly 30 minutes before your departure. (see here: http://www.easyjet.com/en/help/boarding-and-flying/boarding-and-on-board-services )

Customs is irrelevant, takes zero time. You will have to go through immigration (passport control), which can take 10-60+ minutes. You then have to pick up your bags, possibly change terminals and get to the Easyjet check-in counters with at least 40 minutes to spare.
Plus transatlantic flights can be 1 hour late if the wind is in the wrong direction.

Conclusion: No way. Minimum 3 hours for 2 intra-Europe flights, 4 hours if arriving on an intercontinental flights.

I also agree that no way would I do this. Yes, both of those airlines use Gatwick but Gatwick, despite being London's "second airport" is gigantic and has two separate terminals (connected by a shuttle train, but also a lot of walking). Passport control in the UK takes longer than most other countries (and this is 'pre-Brexit'), I have waited over an hour multiple times, it depends on how many flights are arriving at the same time. And as pointed out, delays of an hour or two are very common - weather, delays in departing the US, waiting for a gate on arrival, etc. In fact, when I'm doing what you are doing (Norwegian is one of my favorite carriers, as is Easy Jet and I frequently do that through London) I schedule myself a DAY or two in London. I realize most people don't have that much time but there are just too many possible problems - then you are stuck buying a very expensive same day ticket - if you can even get one - to Milan. Then what about the rest of your trip, sometimes it's a domino effect screwing up hotels and transportation connections.

I don't know what city you are flying out of but have you considered Aer Lingus - they fly to Milan with a connection in Dublin but all on one ticket.

I don't think it's accurate -- at least it didn't used to be -- that easyJet will transfer you to a later flight using the same ticket. Not for free. Most of the budget airlines insist that a ticket is good for only a precise flight; any change is at the customer's expense.It won't matter if both Norwegian and easyJet flights are bought through Kayak or some other agency; they are still separate tickets. Now, things are always shifting around in the low-cost business so I can't be certain. Still, the inevitable hassles with rechecking alone would not be worth it for me.

Easy Jet has a partner ship with other airlines where you can book flights worldwide.
If you book the flight you would use this site:
https://www.easyjet.com/en/worldwide

Read under Delays, cancellations and re-routed flights it says Should you miss any sector of your journey they will re-book you onto the next available flight.

https://www.easyjet.com/en/help/extra-services/worldwide

Be sure to use the easyjet website NOT kayak, or you do NOT get the protection

Below from the easyjet site

How to book

Worldwide by easyJet is available exclusively online, you cannot book this service at the airport, through a travel agent or a partner airline’s ticket office. Go to www.easyjet.com/worldwide to start your booking.

When making a booking individual reservations will be generated for each flight and you will receive separate confirmation numbers for each sector.

I am thinking about flying from the US to London on Norwegian Air and
then catching an Easy Jet flight 2 hours later. Will I have to clear
customs in London? I know I will have to get my luggage (If I check)
and recheck. Wondering if I will have enough time.

Not sure why you're worried about customs; it's a walk through unless you have something to declare. I'm not sure you would go through customs anyway. That all said, 2 hours is not enough given your need to retrieve your baggage and go through security.

Now.....If you have the budget to buy new tickets on the spot (and buy new return tickets on EasyJet back to London n case those are roundtrip between London and Milan), you could always roll the dice. Just keep in mind, if you miss the the EasyJet flight, you're buying new tickets regardless of whether it were one-way or roundtrip. From what was advised upthread, you'll have to go online at the airport to buy those tickets.