Best thing I found for this was either a 12 bar blues in A or E and switch every 12 bars between lead and rythm.

Then what to do is cut it down to 8 bars of each, then four bars.

It's pretty easy that way as if you know what scales to play over it, you'll never run out of ideas and you can start switching pretty quickly between both.

Another good song is I don't know, Ozzy and Randy Rhoads.

This switches very quickly at times between a "chugging" A riff and some pretty snappy lead playing, it's a good way to learn how to quickly switch between solo and rythm.