There is some precedent in the U.S.
The Teamsters have represented pilots, flight attendants and mechanics at various carriers in recent years. The IAM (of Eastern Airlines strike fame) have represented mechanics and flight attendants but no pilots that I am aware of. Most pilot contracts have provisions for honoring another union's picket line but that is sometime selectively enforced.
Years ago there was a strong U.S. flight engineer's union, the FEIA, that became a threat to ALPA's cockpit dominance. FEIA wanted to make the flight engineer second in command and paid accordingly. FEIA went on strike in the 1960's and ALPA cut a deal to make the flight engineer a "second officer" pilot and walked over FEIA's picket lines to break the FE union. Many ALPA contracts still contain legacy language concerning upgrade training for professional flight engineers to become pilots with no loss of seniority. And in the past, I've flown with some of these 1500 pilot hour captains (actually, they could count FE time toward part of the 1500 hours).