Poly Rhythm capable?

Oh, so you don't mean 7 beats in one part in the same time frame as 6 beats in another part, but rather a seven beat pattern and a six beat part that go out of phrase and come back together after 42 beats? For the record, that would be more accurately described as "hemiola", but the answer is still the same - it can't be done in MuseScore, but also, you don't ordinarily notate that by giving people different time signatures. This sort of thing happens all the time in published music, but it is virtually never notated that way. You say it would be much easier, and maybe you meant for you as the person writing the music, but it's definitely not easier for the guy charged with actually keeping the ensemble together in performance, nor does it make life easier for the guy who is reading his part in a different time signature from everyone else. First thing I'd do as a pianist when faced with that unpleasant situation is write in the downbeats that everyone else is reading from, so when we make eye contact or watch the director (if there is one), I know where the *real* beat one is.

And really, it's actually not necessarily easier to enter using different signatures, either. If it's a literally or almost literally repeating part, you can enter those seven beats then copy and paste them in 6/4 just as easily as in 7/4. If the part doesn't literally repeat significantly so you have enter each iteration individually and you find it helps to see the barlines every seven beats while entering the notes, you could create a section in 7/4 later in the score (or on another score), enter the piano part that way, then copy and paste the whole passage where it belongs.

But if you really need to break with standard practice and actually notate two time signatures simultaneously - like if you are positive the people playing the score will prefer that - then you *can* kind of be fake it. Notate the thing as 42/4 (!), then hide the time signature, then add 6/4 and 7/4 "time signatures" as ordinary text, then draw in bar lines where needed. It would be no fun to notate this way, so you'd have to be sure that your layers really did prefer this (and again, it's very rare, so most players would *not* be expecting to see this, and would more likely be confused than helped). But if you needed it done, it would at least be possible.

The only cases where I'd personally consider this worthwhile if the specific figures involved were such that writing it in the traditional hemiola manner would result in tuplets that crossed the barline.

But in any case, because the need does very occasionally arise, I would agree that this is the sort of thing Musescore should probably eventually support - as is the notation I thought you meant originally.