Formatting a page for 2 pieces of music

Better like this? : Sally Ann copy A.mscz

EDIT: Oops... After re-reading (the title especially!), you probably want to display the two pieces on a unique page (and not two) -;)
Second try to come. So, eg: Sally Ann copy 1p.mscz

NB: I added a section break at the end of the first piece. There is a known bug with the playback (when repeat, there is a pause of a few seconds), but this addition allows to avoid to add a start repeat barline at the beginning of the second piece.
And for "the program wants to add a key change measure that I must delete", go in: Style -> General -> Page -> Untick "Create courtesy key signature" (and also "Create courtesy time signature" I think). Possible also to untick in Inspector, but you have to select first the "main" key signature and time signature (ie at the beginning of the second piece), not the courtesy signatures.

You can add a frame to a piece using the Add menu, and resize it by double clicking and dragging the handle or using the Inspector. The Inspector is also how you can set the amount of gap between the frame and the music that follows.

Distance between *staves* (as opposed to systems) is fixed for the score normally, although you can force two staves within a system to be a different distance apart than the others by Shift+dragging the lower one up or down. However, your score has only a single staff, so it isn't actually the staff distance you are concerned with - it's the *system* distance. Go to Layout / Page Settings and set the minimum and maximum to whatever values you want, and MuseScore will automatically choose the appropriate value for each page within that range

You wrote: ...but the program wants to add a key change measure that I must delete.

Since you want 2 pieces of music on one page, using a section break (from the Breaks & Spacers palette) will eliminate the courtesy time signature at the end of the first piece.

Regards.