Walking between coaches on trains

Not difficult, but it is kind of like switching seats on a airplane, if nobody else has the seat reserved, you can. A couple of added complications, One, do it after the conductor has checked your tickets to avoid any misunderstandings, and two, unlike airplanes, the train will make stops along the way and new people will board the train. If you get comfy, someone may come along with a reservation for the seat your in and you will have to move back.

The aisles are narrow and you or someone else might have too much luggage, increasing your challenge. Your original online purchase system might or might not allow you to exchange tickets and request a different seat. Easier than that might be to ask a ticket sales office to have your seat assignment changed, preferably a day or two ahead, if you're in a train station.

I find it odd that if you booked the two tickets together that you would be assigned seats in different cars. If you booked one at a time, then I can see that happening. You best bet is to see a ticket agent a couple days prior to your travel and ask for the seats to be reassigned. It is easy to move between coaches and you need to if going to the snack/lunch car. And sometimes there is not a water toilet in each car. Good luck.

You didn't, by an chance, purchase those tickets via RailEurope, did you??? Purchasing directly from Trenitalia usually defaults all tickets to the same coach or if not, allows you to choose your own seats together.

My experience has been that the ticket office cannot do much. They will direct you to see the capotreno when you get on board. He/She will switch your seats to two available together. We do this if we get seats in a noisy car, or without a window. The capo is quite accommodating.

This happens if you buy two tickets, one at a time. If you purchase one ticket then go back to buy another, you have to check a box "select a seat nearby" and Trenitalia will show you available seats near your companion.

Board early so you can put your luggage together on the luggage rack in one of the coaches. Then go with the flow. If the train isn't crowded, you'll find 2 seats together somewhere. In my experience, people don't get upset if you are in their seat, they just expect you to vacate it. Occasionally, they'll just sit somewhere else.