I actually ran into a very similar problem. I rolled a female aliance paladin. I figured for such a melee role, she would have had an overwealming influence from her father, so in her story I sort of wrote her mother out of the picture at an early age. As it also happened however I had an undead priest girl that I couldn't come up with any back story. *lightbulb overhead* --> priest is paladin's mother who died and came back joining the forsaken.

Cheese? yes. However it allowed me to embellish both characters at the same time. However being on the same account, and different factions, and now even different realms, meeting is pretty much not going to happen. But at some point they're going to pull the plug on this whole world of warcraft thing, so I think I'll leave the final chapter and theoretical meeting until then. It's also a bit more interesting if they could have met each other many times but not known each other.

And also they could sort of interact through a third party (imaginary, or some actual NPC). Probably the paladin's father / priest's husband in my case. Good luck in figuring that all out. :)