When you say Room Overlap possibility, do you mean that the rooms could have the option of being sold either separately or together as one unit?
MyAllocator offers the possibility of paying for only their channel manager service without other features. I don’t think their standalone channel manager can sell the rooms as multiple room types though. Normally you would need a PMS that would map each room to multiple room types in order to do that.
Hopefully someone else here is using a product that does what you need and can share their experience.