I don't know if this is what you want, but you can do a slide-in with KenBurnsEffect. Let's assume you have a 640x480 video:
Code:
a=AviSource("clipA.avi")
b=AviSource("clipB.avi")
StackHorizontal(a,b)
KenBurnsEffect(startFrame=0, endFrame=29, endPanX=0, startPanX=-640).crop(0,0,640,480)
I think it has to be RGB32, so you may have to add ConvertToRGB32()