You do this in Media Studio Pro using either the crop filter or you can use the Picture in Picture filter, Pan and Zoom filter or one of the other moving paths filters. Plenty to choose from.
I posted a tutorial on how to do this in the "VIDEO Product Tutorials" forum. It's for VideoStudio 9, but I'm sure the technique will work for MSPro as well.
Just overlap both sources (one on Va & the other on Vb) for the duration of the split screen then apply the Slide-Wipe transition. Set the first keyframe to a transition degree of 50%, copy that keyframe (context menu) and paste it to the last keyframe.
POOF!! Instant split screen.
In MSPro 8 the same idea goes, but with single track you'd literallly overlap the clips full length and use the Slide-Slide transition.
Because VideoStudio is a single track editor the procedure would be similar to that of MSPro 8.
Using Slide-Wipe Transition won't work for VS because you can't set a transition degree or keyframe, just border, color, soft edge, and direction. However, I'm not familiar with MSPro, so I was just using my knowledge of doing the same task in VS9.