Hello UR...
The 721 will (most likely) need to have 2 separate lines connected to it to drive the 2 tuners. Those lines need to be 2 separate lines from your switch, or 2 separate lines from your twin or quad lnb. If you have a twin lnb, you have either 1 or 2 cables coming out of the dish that run directly to your receiver(s). There are no switches connected to them unless you are cascading in a 2nd dish pointed at 61.5 or 148. If you have a quad lnb, then you have up 4 lines coming out of your dish straight to your receiver(s). If you have 2 dual lnbs, then you have 2 lines coming out of each lnb (meaning 4 lines out of dish) connected to a switch or switches, and then lines coming out of the switch going to your receiver(s). Also, if you take a close look at your dish, you will be able to tell if you have 2 lnbs installed, or 1 twin or quad installed. If you upgraded several years ago, you definitely don't have the quad, and most likely don't have the twin.