Coaching two positions at the same time

First off, like you said, work on transferable skills first and foremost. You could even cross train a couple guys too?

But if there is somebody else capable and free I would suggest asking them to help out but being very clear about what you want them to work on. Even if it were somebody who doesn't know much about LB play, if you give them clear expectations and show how the drill runs they should be able to handle it.

I've done this when I've coached the entire OL and TE's at once (e.g. pulling drill for guards and working double teams with everyone else, kick sets for tackles and interior slide pass pro, etc) in order to keep everybody busy. It definitely worked best when the JV OL coach could come up for that one period, but I also allowed seniors to run a group here and there.