I would wait at least a few days to a week to put in the new one to see if it seems healthy (doesn't lose limbs, etc) even if it is a new cage. I'm sure plenty of people don't and it works out fine though.
The main problem I see is most pet stores don't keep them at proper humidity and temperatures and if you suddenly put it in a dramatically new temp/humid enclosure it might kill it. I've had this happen with a gorgeous tarantula worth a lot of money once. The previous owner did not know how to take care of it and when I gave it proper temp/humidity I did it too quickly. That was a very sad day for me. Tarantulas aren't hermit crabs but they are invertebrates and can have this same issue. I'd like to say my care has always been spot on, but that was definitely a mistake I'll own up to.
Edit: However, so long as you monitor everything do what you think is best. Getting your hermit crab another friend is definitely better than having a single IMO. I'm not an expert, just a hobbyist mentioning what I've observed over the yrs around arthropods.
His buddy passed about a year ago, and I've thought about getting another. Any tips on introducing them to each other?
