Beams in 7/8 time

I would create a new 7/8 time signature in the master palette, and set the beaming as 12,12,123. Since the default 7/8 beaming is already 123,12,12, you can insert the default 7/8 sig for the first of your two-bar pair, and your newly-created 7/8 sig (12,12,123) for the second. You'll then need to alternate the 2 sigs for as long as you need the alternate-bar beaming structure.

It's possible there may be an easier way to do it, but if so, I'm not aware of one - perhaps someone else may have a better suggestion? Anyway, in the meantime, I hope that helps.

The way the Beam Properties palette works, you have to click the head of a note whose beam you want to change and then double-click the relevant property in the palette. Difficult to use? Maybe. Impossible? No.

To change a 7/8 measure from 123,12,12 to 12,12,123:

1. Select the two notes that begin the second and third groups (by first clicking on one, then holding down Ctrl or Cmd and clicking on the second) and apply the "Beam middle" property;
2. Select the two notes that you want to begin the new second and third groups and apply the "Beam start" property.

I suggest that you set up shortcuts for "Beam Start" and "Beam Middle". Then you can whizz through the score note by note and set the beaming more easily than by mouse-clicking. Too much mouse-clicking (IMHO) only leads to accidentally moving notes and other elements.

I have Beam Start set to ` (the top, left key on my keyboard to the left of the number 1) and Beam Middle to \ (the bottom, left key to the left of Z). Try it and you'll likely soon get the hang of it. Money back if you are not delighted.