I didn't really do pair-ups in the first half of the game because it usually ended up with my troops not covering enough areas of the map, and the one weak guy/new recruit gets ganged up and killed. Though, I stapled Frederick to Sumia a lot to make her tankier, because I wanted to train the aerial units fast (and those two ended up marrying first).
I'll definitely make sure to plant the avvy next to Lon'qu as much as I can on the next playthrough; it should be a lot easier now that I have DLC.
But for now, Lucina's pretty much like a second Morgan, so I can't complain. ^^