MythTV Multiswitching HowTo
Dean Pemberton has produced an excellent write up on how to use a DVB-S Multi switch with MythTV. You can find the details at the main MythTV Wiki.
These multi switches allow multiple DVB-S tuner cards or satellite receivers to share a series LNBs each of which picks up signals from a different satellite. This is idea for sharing dishes between a multi-dwelling environment or where you have a lot of tuners wanting to share access the same satellites without potential conflicts.