To answer the question you asked, yes, you can use an SA 8000 and a HD Set top on the same tv. Just connect the 8000 to the svideo/composite/or RF input, and the HD set top (3100HD likely) to the component inputs. Then just switch inputs to view the desired box.
Because both boxes use the same remote codes, a press of a the remote will send the same signal to both boxes, which isn't a big deal unless you're ordering iControl or a PPV movie, and then you might accidentally buy two copies! Just turn one box off before doing so. (You can always call customer service and ask for a credit if you get double billed).
Hope this helps.