We have a good 5.5-6 hours in the area and were planning to just catch pompeii. How long/distance between two/what sort of transportation would be available for touring both sights (Herculaneum and Pompeii), and is this feasible? Thanks.
You can EASILY spend five hours in Pompeii. It is massive and exhausting. If you had a whole day, you might try for both. With the time you have I would stick to Pompeii
They're a short distance apart by commuter train. You could pay an abbreviated visit to both in that amount of time, depending on where you're coming from.
If 5.5-6 hours is the time you'll have available to tour the sights, then both might be feasible - say 3 hours at Pompeii, 1 hour of travel time and 1.5 hours at Herculaneum. If it includes travel time from Naples train station or some other point, then visiting both would be rushed and your better bet might be to just visit Pompeii. Plan on a 30 minute Circumvesuviana train ride between the 2 sights (2 trains per hour). Entrance to Pompeii is only a few minutes walk from the train station, but Herculaneum is about a 15 minute walk each way.