Not sure if they fall into the "pairs" catergory, 
but here are two cels I have in my collection 
of "Gundam X" that go together, literally! I got 
the one of Olba first, for obvious reasons 
*drool*, and a couple weeks later saw the cel of 
his Gundam Ashtaron up on Japan auctions. Not a 
big fan of the mecha so I wasn't real interested 
in getting it, but a few days later I was 
rewatching the series, and spotted the Ashtaron 
cel -- as a background cel for the Olba I had! So 
I *had* to snatch it!! When I received the Gundam 
cel, the background code/number on both cels 
matched, confirmation they're meant to be 
together! And they shall forever stay that way 
with me!! ^_~

If anyone cares to see it, click the url link 
above to see the screen capture from episode 19, 
and you'll see both the cels at work! Gotta love 
it!!