At this point, most builds have moved away from two gunners, and you just about never want a second person joining midmatch as a gunner. That's not saying you won't do good with two gunners, but it's going to be a lot more difficult than having two engineers.
That being said, there's never a reason to have a second pilot.
I think it's a good idea. I rarely have a second pilot join, but I get a second gunner a lot. Usually it's newer players who aren't aware that almost all captains prefer a 1 gunner maximum on their ships. So a warning about them joining as a gunner might not have any impact on them.