I went back and re-read just to be sure. OP posted about a FP runner who then joins his party who is already in line.
I'm trying not to be confusing, but I think I've got it straight. I *think* you're talking about Family X going on a ride without FP Runner. FP Runner gets the FPs to an attraction while Family X rides. Then Family X and the FP Runner meet at the next ride. Right?
And I thought (like you) that I read a reference to guide books in the OP, but now I don't see one.

At any rate, I don't know whether guidebooks suggest this or not. I seem to recall the Bubba Swap (is that what it's called) from the UG
did require cutting in line. Anyone remember this as well?