I hope you are able to get one and sell the 550D.
Personally, I chose to keep my old 60d with 150k+ shutter cycles when I purchased the 6d because it's not entirely an upgrade - the 6d lacks the swivel screen which is great for tripod, and the crop factor is very handy for tele (narrower fov) and macro (less dof). Last not least, not changing lenses but having two bodies is useful sometimes and you always have a failsafe if your 6d should ever break.
If your 550d isn't in pristine condition anymore you might also think about keeping it because the resale value probably probably isn't too great nowadays, the 18mp sensor is aged and newer models have many more features.
I agree it's nice to have two bodies, but if you really don't need the backup body, or a swivel screen, and can use the money to help fund the purchase, it does make sense to sell. I sold my 50D for a fantastic price right after I bought the 6D. And unlike your precious 60D, the 50D had AFMA...so there!
I'm actually considering buying a Rebel T5i (I forget what it's designation is over there, maybe a "60million D"? Joking!) The reason I'm considering it, is because although it lacks AFMA, it's probably good otherwise, has that magical swivel screen, and refurb units have recently been on sale for $419, body only. I sold my 50D for 50% more than that! If I can't get one for close to that price, then I'll just wait till I can, even if it's a year from now.
Since nobody answered my other thread, what's your favorite pocket size camera, Marsu? Or maybe you're not opinionated enough to say? I might have to coax it out of you!