It can be done using a DiSEqC switch, a friend of mine used to use a TM600 with a motorised dish and a fixed dish with 3 LNBs through a four way DiSEqC switch.
Can't remember exactly how he did it, but if i recall, you have to store your sat posistions using DiSEqC 1.2 settings, rather than usals.
Then in sat configuration menu there is a a 'non standard user defined configuration' (will be different on a DM800) to set it up.
A bit of googling should find the correct configuration for a DM800,though
EDIT
Just found this with a bit of Googledge.....
Code:
On your remote
Menu-Setup-Service Searching-Tuner Config and select DVBs2
Set configuration mode to Advance
Scroll to the satellite selection line and use the <> buttons on your remote to select a sat position.
Now Scroll to LNB and use the > button to select LNB1. Repeat this for all positione your motor is using.
Then for the fixed dish sat position select LNB2.
Google 'DM800 motorised and fixed dish' there is loads of info out there...
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