Be careful! Some hotels, when you ask them to keep the room, they combine the folios without telling you, and in that case, it's all treated as one stay, not multiple stays.

Hotels have a lot of people asking for payment to split on a single stay, so that fact it's booked differently and paid differently does not mean they can't combine the folios.

I'm not saying it happens all the time, but you have to be aware that it can happen, and that if it does happen it may be too late by the time you realize it.

It's safest if one person checks out and then the other person checks in, without both people present at the same time, so that it's harder for them to do that. But to do that, obviously, you have to get everything out of the room when you check out, and you're not likely to get the same room the next night.