They do both depending on need. Both have their upsides. If the Captain has been on the airplane, that's a plus. However, he's probably not going to want to study flows with you, etc ....

I'd prefer FO-FO for initial since you can concentrate on FO stuff and help each other. After you are familiar CA-FO works perfect.

On some aircraft (like the 737) there are Captain specific toys (like the Heads Up Guidance System) that take a lot of time to get through all the possible permutations while you sit there with nothing to do.