Yes, it can be done. I'm running two slingbox's, but in a different way than you are proposing. I've got one connection coming out of my DVR through component to one slingbox and the other connection coming out of my DVR from composite to the second slingbox. Using it as a backup in case the first slingbox fails.
This thread gives you info on the networking side to setup the two slingbox's on the one router.
My only reservation is not knowing how DirecTV protects its Sat box's using HDCP. If you've got the HDMI connection getting used for the TV, then I'm assuming that the component connection on the box has been blocked leaving composite and S-video for the two slingbox's. Are you talking about using something like this
......where the signal out of the Sat box goes into the input and get distributed out to the composite connections on both slingbox's?
Other than requiring a fast enough upload connection for two streams and seeing two sets of I.R emitters on top of your Direct TV box forcing a potential channel change fight from your two remote viewers, I don't see why you should have any problems in doing this.