I use both at weddings, but mostly I'll use one or the other, I'm not switching bodies a lot. For the ceremony, I'll switch back & forth a little, maybe the first dance. But most of the rest of the time I'm using one body or the other primarily.
I do like having both because there are strengths to each camera. The 6D is lighter and more 'nimble' to use, so I like it for the reception when I've been shooting for many hours, it helps reduce fatigue. Also, the low light AF is so great, I can get accurate in-focus shots in practically no light at all. Just this weekend, I was at a reception at a private residence, people were all around the house, many in the lower level where there was little to no lights. I was following the bride and groom around using the 5D3 and it was having a hell of a time locking focus. I was finally able to switch cameras, and suddenly the 6D was instantly locking focus where the 5D3 would hunt for 5-10 seconds. It's literally night/day difference between those two cameras in that situation.
I do wish they had put dual card slots in the 6D, though, it was the one feature that made me think I didn't want to bother with the camera. I'm glad I rethought that, though, it is a great camera.