I've come across a number of TM pumps that are of the split design; as I understand it, they're used to pump two chambers to different vacuum levels using 1 pump and are typically found in mass spectrometers. As I see it, the input to the lower (relatively) vacuum stage could be blanked, thereby creating a single input pump pumping to the higher vacuum level that the pump was originally used for.
Is there anything wrong with my reasoning?