So Roark Garnet can boost a pilot's skill to 12. If the boosted pilot has swarm tactics, can a second pilot be boosted as well?

Yes, you can chain Swarm as well if you have multiples.

If you've got a bunch of pilots with Swarm Tactics, it can really ruin you opponent's day.

Roark with ion turret, and 4 green squad a wings with refits and swarm tactics. 5 rebels with pilot skill 12.