is it possible to go to capri and positano in the same day>

Where are you staying?

we are staying in salerno

I would say no. Getting to Capri takes some time; for us, using a ferry from Sorrento, it was close to an hour. You land at Marina Grande, but that’s just a place for boats to dock. You still need to get up to Capri town or Anacapri. At the end of your visit, you need time for the reverse. The good news is that you won’t miss much if you skip Positano. It looks charming from the water; but on the ground, it is mostly tourist-oriented shops, offering over-priced clothing and souvenirs. If you go to Tripadvisor and click on “Things to Do,” you will see that there is plenty to fill a day on Capri. We found ourselves wishing we had spent a night there.

Are you planning to use public transportation for these two places or will you have a car? What time of year? Will the tour to Pompeii drop you back in Salerno at 2:00? If so and you're there when it stays light into the evening, my suggestion would be to take the ferry or bus to Positano that afternoon and stay until evening, have a nice dinner in a waterfront restaurant and watch the sunset before returning to Salerno. The next day I would plan to spend the day going to Capri. If you do both in one day it will be an exhausting day and you might feel short changed, especially for Capri.

we take the train to naples for the Pompeii tour which will be from 10-2. we will have to take train back from naples to salerno.
we are going next week. we have a rental car but everyone says to use public transportation.

Capri, No. Your question is much less meaningful without knowing how much money you are willing to spend to make it happen. Pompeii is not missable. Positano and Sorrento are, but one of them is desirable.

If you insist, the bus to Salerno originates in Amalfi. You could pay for a taxi from Pompeii to Amalfi (as a weaker substitute for Positano) and bus to Salerno. Starting in Positano with a bus that terminates in Amalfi will add a lot of time to the trip.

On the plus side, Amalfi has much less of the useless international luxury boutique chains that infest Positano and Capri. If you get a whole day for Capri, you can skip the AC. Does the Capri ferry from Salerno run in March???

we are going next week

In that case, if the weather is good, I would skip Positano and spend the following day going to Capri - if it's a must see for you.

ferry doesn't run from salerno to capri in march; it begins in april

The ferry does run from Naples to Capri year round. So if you really want to visit there you'd have to train to Napoli Centrale then take the Alibus to the port. Check all return timetables before you commit.

Have you read responses to either of your previous posts about Capri from Salerno? Ferries do not run to either Positano or Capri from Salerno in March. You'd have to take a SITA bus to Positano, a bus to Sorrento, a ferry from there to Capri, and then repeat the process. Or go to Naples by train to take a ferry to Capri from there. You really cannot do both Capri and Positano in 1 day during off season.

https://community.ricksteves.com/travel-forum/italy/what-is-the-best-way-to-get-to-capri-from-salerno-in-march

https://www.positano.com/en/ferry-schedule?from=salerno&to=positano#

queen,

While it may be possible with carefully planned transportation connections, it wouldn't provide an enjoyable visit to either place and would likely be exhausting.

I'm not sure I'd want to spend a full day wandering about Positano, as there's not really much to do. At that time of year, laying on the beach probably wouldn't be an option. If seeing Capri is a priority, Ferries might be operating from Sorrento which would be a bit quicker than going from Naples. I'm not sure what parking is available near the ferry dock in Sorrento?

As you'll be using a rental car I assume you're aware of the requirement for each driver to have an IDP, and that you're aware of ZTL areas?

While it may be possible with carefully planned transportation connections, it wouldn't provide an enjoyable visit to either place and would likely be exhausting.

Ken beat me to it.

IF you can string together the transportation logistics, I doubt it will produce an enjoyable experience.