Quote fromI guess they could both be counted tword end boss but fight them seperatly. Perhaps they both are trying to undermine each other. Belial could appear as a NPC and lead you on about him to try and get you to kill him and as you kill Azmodan he then trys to kill you or something along those lines.
This sounds like a great idea. I could definitely see Belial tricking you into believing you are helping an innocent man out to help rid of Azmodan(because Belial himself couldn't do it alone & likewise that Azmodan couldn't rid of Belial) and as soon as you destroy Azmodan **the ground starts to tremble and the veil of ignorance Belial once had over you is lifted and he reveals his true form to now bring you asunder!** but of course we would vanquish the demon and take his dirty loot! AARGHH!