Enter the notes as you want them to appear, starting with the bass clef. Mute the notes that are in the treble clef and toggle "no ledger lines". Enter the notes as you wish them to sound in the treble clef, and hide them. N.B. If you change the spacing between clefs you will have to realign the the seen notes in the treble clef.

At the time being, you can't. The notes belong to one staff only, and the beams link notes belonging to same staff.

However, using Carl's advice, you may obtain a visual effect similar to typical piano scores.

Don't forget though that pianists can understand "octave shifting" or clef changes, so you may not need the beaming you ask for.