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.
First, check out my answer to "What is a double-stuff breaker?"
Second, take a quick moment to educate yourself on "rule of six" panels. I am fairly sure you don't have one.
single pole vs tandem locations
Generally, common 1-pole breakers can go anywhere in the panel (except in Rule of Six panels in the top section, because that would put you over six).
Double-stuff breakers can only go in spaces designed for double-stuffs. A typical panel will allow this in 0, some or all of its spaces -- though "all" is becoming rare. The labeling on the panel will say where they are allowed. Newer panels also have "keying" which makes the wrong breaker impossible, though older ones do not, and there's always the wiseguy with a file defeating the keying.
do you use single until you upgrade to tandem?
Tandem is a downgrade. The breaker will perform more poorly and high loading on one side will make the other side more susceptible to nuisance trip. Much worse, double-stuffs are not available at all in AFCI or GFCI, and virtually every residential circuit today requires those. What's more, remodeling must be to new codes and that requires more circuits.
Double-stuffs are the mark of a cheap builder who decided to save $30 by buying a 20-space panel instead of the 42-space the house actually needs.
what does "circuit breaker was oversized" mean?
The breaker's job is to protect wires and devices. 14AWG wire needs 15A protection or it will overheat. Common receptacles need 20A protection or they will overheat. Certain devices need protection of a size they will specify.
An oversized breaker means somebody made a mistake, or swapped in a too-large breaker because he was overloading the circuit and got sick of the breaker tripping. Replace the correct breaker. Breakers are $5.
what's a normal breaker really called?
It's more defined by the absence of a label like duplex or quadplex. To be more specific, call it a 1-pole breaker or 2-pole breaker.
general rules?
Yah. Only use breakers listed for that panel. I certainly hope your panel is a Siemens/ITE because almost all your breakers are listed for those panels.
The Square D Homeline breaker in position 17 is the snake in the stick pile.
Now a lot of people go "oh, but they fit". Those people aren't qualified to decide whether they fit, that's UL's job. There are several brands of 1" breaker that will mutually snap in, but if you have any experience you'll notice the insertion force is all wrong. That's because the busbar blades are each shaped differently, and each breaker is made to mate with its family's busbar and not the others.
There is such a thing as a "classified breaker" made by one brand specifically for another brand's panel, but if square D made such a thing for Siemens, they would never market it as Homeline (HOM).
Best Answer
It's vitally important you know what a MWBC is and how to recognize one, when you are doing things with existing circuits and quads/2-poles. Placing both legs of an MWBC on the same pole Is Very Bad and can start a house fire.
That said, yes, 2-pole breakers in general can be used for two independent circuits.
However, in renovations you typically need to build to modern Code. It's very common for people to do a reno and go "Aw, I'll just re-task the original wires that came here". Nuh-uh. Your kitchen reno needs to comply with modern Code, and that includes wiring, and that includes AFCI/GFCI breakers as required. And where AFCI/GFCI breakers are required, that precludes use of double-stuff breakers. Which is going to be a problem in a tight panel. Code says "too bad; upsize or subpanel".
And yeah, that may be a good idea. If you can find a suitable mounting point (e.g. facing a pathway so that nobody ever blocks the working space), adding a subpanel to the kitchen reno makes a lot of sense. A well-powered kitchen needs 8 circuits or so. That lets you remove all existing kitchen circuits from the overstuffed main panel and substitute with just 1 2-pole.
Now, we strongly recommend wildly oversizing any panel; so if you just heard "8-space panel" what we meant you to hear was "30-space panel". Spaces are dirt cheap when you're buying the panel. And they save you a fortune later when you need to add spaces.
Not lightly do I spend your money, but please - spend $60 extra on another 20-30 spaces. It's the best investment you can make in electrical, bar none.