I've got an 8 month old boy and girl and took them together. Some people say that you should not separate them even for a couple of hours as they may reject each other when put back together.
But I took the Boy by himself to be neutered, and they were also kept separate for a week while he healed and they went back together fine.
Do whatever is best for you.. financially too of course. (some people may need to spread the cost)
I think its easier to do them together cos then there's only 1 date to remember when they're due again!