Fenders that will work for swapping between 650b and 700c wheels
Great idea - smaller 650B wheels with winter tyres will be easier to swap than swapping tyres on your only set of wheels.

As long as your fenders / mudguards are wide enough to cover your wider tyre, and large enough to not touch your larger tyre, then they'll work fine for the smaller and narrower tyre.

Note that we consider specific brand recommendations to be off topic.  Most Cycling shops have the well-reputed brands who will have suitable models.   If your guards are rated for CX tyres with ~43mm tyres in 700c (ie, etrto size 622 wheels) then it will work fine for smaller wheels, but with a little more gap.

Another option is to get two sets of clip-on fenders and change them at the same time you swap wheels. Though this could get tiring quicky.