The one I had shot 100% high at the lowest settings, and I could never get used to the high rib. Someone else now owns it. I would see how it patterns, and if you can adjust it to shoot how you want it to. I always set up my guns with the more open choke in the bottom barrel, then choose to shoot the barrel with the appropriate choke for the distance.
Lower barrel first in doubles. Don't use the O/U for anything else (combo gun). When patterning the O/U, the top barrel was essentially flat, the under barrel was 1"-2" higher at 13 yds. The gun is still a lot better at doubles than I am.
I am shooting my upper barrel first all the time. And in fact for doubles it might make even more sense..........Because I shoot a lighter load out of the top barrel....and what is a full strength load out of the bottom............relieving the most felt recoil...........
I shoot a 67 Broadway. Always used under (mod) and now went to using the top (full) first. Scores went up using the top. I can feel the recoil a little more in the top but I seem to be able to score higher with the top. Feel like its all in my head but I am more confident with the full vs the mod.
If you have a Citori Trap Plus. it will have adj. rib. stock. comb. ported barrels. If so the bottom barrel has about thirteen or so ports with the top barrel having about nine ports if I remember correctly. The reason the bottom barrel has more ports is to minimize barrel jump. and the top barrel less. so therefore that will be your barrel for your second shot. BOTTOM BARREL FOR FIRST SHOT.