I've travelled with all 3 of my guides. In Australia we get a free seat if flying with a guide dog. Generally they get the floor beneath the window seat, and I tether them to my seat belt. I've always been able to angle myself to provide a buffer. Also always take the harness off both for dogs comfort and not to break the harness given the tight space.
Regarding the question about multiple guide dogs on a plane - we used to have a rule that only 2 service dogs per flight were allowed, but the rule was flexible if the airline notified early.
I prefer the bulkhead as my guides are larger and don't fit easily between seats - it can be done, just isn't as comfortable.
Dog tax:
$1 my second guide, Zeke. The plane was particularly cold so he got my coat.