I just had another thought -- don't know if it'll work or not and I cleaned out the trial stuff so don't have it to play w/ at the moment, but:
If you were to use both sets of handles but one with blank text, maybe you could get both symbols one below the other with just the one text line...something like
As said, not sure if can get that to happen or not, just a passing fancy of a thought...
Well, I did try it and, by golly, it does work. Not quite what you were looking for ideally, but not too awful bad I'd judge...see what you think w/ this--
[h_leg,h_obj]=legend([p1(:).' p2(:).' p3(:).'],'p0',' ','p1',' ','p2',' ');