With Magic Keyboard attached to iPad I do the following:
- select Write mode in a piece that has music in a 4/4 Time Signature
- select an item that belongs to the treble staff only of a solo piano and start the note input caret.
- Press Shift-M
- type in 12/8 into the Time Signature popover
- press Option+Return
Result: the 12/8 Time Signature is created in both Treble and Bass staves of the piano. (Same results when tried with a single staff instrument. The new Time Signature is created in all staves.)