so the first shot is the bus, and the second shot is revealed from behind the bus as it goes past?
if I know what you mean , I would use a track-matte for this, it will probably involve some fiddling around with key-frames but it should be OK if it's a steady shot of the bus...
I'd be using Premiere 6.0, I assume you'd be using another version, but if you search the help for Track-Matte it'll tell you what to do.
make an image of the first shot with everything you want to keep in white and what you want to be transparent as black, (or the other way around maybe )
import it to your project
tell Premiere that you're using that layer as a track-matte for the bus layer and position the second clip to be revealed under it. (don't know about other versions so look in help)
edit the track-mattes motion with key-frames so it covers the relevant areas of the shot throughout the transition
If that's what you want to do and you'd like more detailed instructions for Premiere 6.0 just ask, if not, someone will probably suggest a good way of doing it soon.
Panasonic SD9, Panasonic NVGS-75, Canon MV600i