You'd need two Slingboxes and if you want to allow viewing two different channels (ie a different one at each location) concurrently you'd need two cable/satellite sources so one Slingbox connected to each.
You'll need enough upload bandwidth at the location of the two boxes to handle both at the same time. If you want HD you'll need a minimum stable bandwidth of 3-4mbps for each so 6-8mbps. Call it 8mbps to be on the safe side. That's for HD you would need considerably less for SD.