I have been slaving a DVBWorld USB box off of a Coolsat.
As the others said, you are good to go. That is, if you want to blind scan OR watch HD.
Similar to the type of situation cracklincrotch mentioned: Another thing to keep in mind is the polarities must match when watching on one box while scanning with the other.
If I am watching on V polarity, I must restrict scanning to V polarity. Otherwise, the slave box will be "disconnected" when the blind scan changes from V to H polarity.
The method Iceberg brought up would be much more flexible. But I believe you would need a dual output LNBF too.