You can mix different MPEG-2 files on a timeline in DVD Studio Pro. As long as they have companion video and audio files, you should be able to lay them out sequentially on a single timeline.
For the sake of proper playback, I would not recomment one file done at 8.5 Mbps rate and the other at 4.5 Mbps rates. Mainly because the end user may not be able to play the file encoded at the higher bit rate well.