We will be in Positano for a week in September. Is it a doable day to do both sites in one day? We are two fit mid 60 year olds but want to enjoy the experience without feeling like we are on a forced march!. Also, how is the best way to handle the logistics.....train, bus, ferry or a combination of all three???
Positano is poorly situated for daytrips without a car and driver. Although both sites, especially Pompeii, can take several hours, you could limit your time at each site, and use the low-cost commuter rail that connects them. The bus to Sorrento, where that train terminates, will slow you down so much that I recommend hiring a car to get to Pompeii at opening time (to save time and avoid high sun ... ) to start the day. You can have dinner in Sorrento and take a taxi home, or check how late the bus runs.
I would do two days trip: one to Pompeii, and one to Herculaneum and the archeological museum in Naples. Both in one day is a bit of an overload, IMHO, and especially so coming from Positano. Take the train from Sorrento in both cases.
I also like the idea of hiring a car to get to Pompeii at the earliest entry time to avoid the heat of the day. Then lunch and Herculaneum in the afternoon before returning to Positano. We did both in one day from Naples, so we were a little closer. But with an early start visiting both in the same day is very feasible.