Combining Circuits to have common powerline
The only situation I can think of where this might be an issue is if either or both of these circuits you want to combine is/are part of other multiwire branch circuits, meaning their neutral is shared with another circuit's neutral. If they both have their own exclusive neutrals or are already sharing a neutral in a mutual multiwire branch circuit it shouldn't be an issue. 

Oh, one other thing just occurred to me. You mentioned a basement wall outlet, but didn't mention whether the guest rooms are also in the basement. In any case, if the basement circuit happens to be protected with a GFCI breaker, and/or the guest rooms are protected with an arc-fault breaker you could have issues. Current NEC codes require arc-fault protection in bedrooms (as well as many other living spaces) and gfci protection for outlets in unfinished basement spaces. You might have an issue there. If only one of those I've mentioned is the case, be sure to combine the neutrals as well by pigtailing them together into the return lug on the breaker. And be sure not to circumvent the NEC's safety rules for either type of location. There is a dual-purpose AFCI-GFCI type of breaker manufactured to take care of both at once, but they are around $200 where I live.