Because PLD only has one full combo.... Sword Oath + Rage of Halone combo. Fight or flight adds a good deal additional damage, but you should be wary of using it if you're right behind the mt in threat.

As for gear: for two tank content that only requires one, fights like Ifrit HM, Titan HM, and certain parts of CT (but not all), you could go full STR accessory to help damage but most body pieces are still going to limit you to only tank gear. Pieces that are "Disciple of war" (which stop appearing at about ilv60) aren't going to have as much STR than higher level tank gear , i80+, and any off stats gained would be hard pressed to outperform the additional STR, AND the tank pieces will also have damaging secondary stats since the only tank-exclusive secondary stat is parry. Any content where the OT has a particular role, Garuda HM, Garuda EM, other parts of CT, Castrum, etc. you should still go with tank gear because you'll actually have something to tank. Crafted accessories is a more complex deal (I'm mostly talking about dungeon and and tome gear), and I can't give advice on that besides put materia on them.

The biggest issue is fights like Ifrit HM and Titan HM where the second tank has nothing to do, which is why often in PF they only ask for one. In those cases, You will generally not get shield swipe procs, and the Riot Blade combo does quite inferior damage to the RoH combo. If you're so close behind the MT in threat that you're worried about pulling threat with RoH, (assuming you are in sword oath) why is the other person MT? WAR has useful non-threat combos, but should easily be ahead if in defiance.

tl;dr sword oath + rage of halone combo, same gear unless you're not actually tanking.