We do this with regular shared web hosting. Both brands have been in operation for over 5 years now and going strong.
We do not "mix" the two brands in any way.. so one brand does not know about the other. They are basically operated as two completely seperate entities, that just happen to be owned by the same company.
This is what we were thinking. Our first go around with budget hosting was a disaster. We ended up cannibalizing $80K/yr of our annual revenues from premium hosting into $30K/yr annual revenues from budget hosting. It wasn't anywhere near all of our revenues, mind you, but it was certainly an ouch.
Thinking our premium customers knew the difference between a budget offering and our premium offering was a big mistake. Many of our bigger customers just assumed it would be the same great premium service with a smaller price tag and switched. When things with our budget offering weren't as perfect and high touch as our premium offering we got a bunch of former premium customers cancelling from our budget offering and beginning the long pilgrimage of bouncing from budget host to budget host.
I knew about ThePlanet, but even a budget dedicated server is quite a step above budget shared hosting. I wasn't sure how applicable their model would be to shared or VPS hosting.
It sounds like it is quite challenging, but can be done with proper management.