Pretty self explanatory, if I trade my Tornadus from ORAS to Ultra Moon, can I have it in my party and try to catch Landorus, or does the Tornadus need to be from Ultra Sun?
Yes, it doesn't matter if the other legendary is from another gen, otherwise it would be very unfair if Game Freak decided we could only use same gen Pokemon. The game just checks to see if you have both a Thundurus and a Tornadus in your party.