Hi mysticeagle : )

I do spilt scenes using Adobe Premiere with 2 finshed rendered scenes in overlaying tracks., and I know Sony vaga is very easy to accomplish this effect. with any movie scenes using the left right panal changes.  so I think the easest way would be depending on the movie editor program your using. I know you can't do it wilth windows movie maker though.    and doing it trough poser will use a lot of your computer  CPU & memory resources. so if you have 6 or 8 gigs of ram you should be alright..  But for me I would create it in a movie editing program. 

Good luck and please be sure you post your finsh film here so we can see the results :)

warm regards

Ivy