Weird connected staff

I'm not sure what you mean - are you referring to the barline that connects them? That's correct - instruments of the same family in an ensemble score are supposed to be connected in that way. it's an important clue to the reader of the score as to how the instruments are organized.

On the other hand, if you have some unusual special reason to want to break this rule of notation, MuseScore does allow it. Just select the barline on the top staff, then drag the lower handle up. But again, only do this if you have a special reason to, as normally this needs to be there.