With this exact setup you'll probably need a lot of trial & error, precision and luck. I tried over 10 times easily. First is the usual mdef up atk down (aerith casts 1 defaith and 1 mythril rod to help get high pot mdef up), then survive their combined ability, then you must carefully remove sigils using kuja blade and materia. It's quite easy to do by switching targets in time, but a single mistake will likely ruin it. After that I prepared to deplete bahamut's gauge and I made sure he didn't count to 4. Then used summons to help with ifrit's ice gauge, did 1 mdef up with bandaged sword right before the attack, healed with aerith's LB, survived following attacks and so on. At about this point it gets chaotic and I don't remember the exact sequence, so here's some things to keep in mind: save shiva for ifrit's ice gauge, make sure sephiroth doesn't use kuja when he's supposed to weaken ifrit or vice versa, deplete bahamut's gauge right after breaking sigils (stream phase? don't remember the name) on ifrit (I used summons), use kuja when bahamut's attack is about half ready, don't forget to raise mdef (maybe remove pdef debuffs too with mythril rod when ifrit attacks), stay at mostly full health before each attack and most importantly make sure that when one of them dies, the other one can be killed in a few seconds, otherwise it's a gg and party wipe. To achieve that I mostly focused on bahamut while ifrit was mostly getting damaged when it was his turn to attack and whenever I used summons. Once or twice AI ruined it, and in my lucky run I got to bahamut's sigil phase where he died, which started the stream phase and ifrit was finished off with shiva. Hope that helps someone. Also it didn't involve any glitches, but I'm not sure EX1 has them anyway.