I think double rocket or maybe double grenade launcher is harder to nail then rocket + rail. I even done it today and i did not feel it is very hard to do.
I must say movement is really important in vq3, and i love to see good players move. Movement does speed you up drastically in vq3, and is really important when it comes to timing items and going from one spot to another. You can do a lot just by knowing movement tricks in vq3 and without even thinking about aim.
But again movement + aim + timing + tactics....those are all important. You can stress one more then another, but playing without one will do nothing good