Dish uses multiple satellites to support their channel lineup. Do you have a single Dish 500 system , or multiple dishes? Signals emanate from satellites at 110 and 119 degrees for most of Dish's channels. Hi-def and local lineups are on additional satellites, I believe.
Do you have two separate dual LNBs with switches, a single-unit dual LNB, quad LNB, or ??? Last year when I moved, I had to get a different switch to support up to 4 receivers. I originally had a Dish 500 with a single LNB unit that only supported 2 receivers. The new one is a 4 receiver system. So I have 4 coax cables come down from the dish.
However, it depends on the equipment you have. Dish Network does have a lot of different LNBs that are dual, quad, etc. and they all look pretty much alike. So first you need to know what is on the dish and then you can figure out what's needed for more receivers. In some cases you can use external switches, in other cases, you cannot. Since you started with 2 receivers, you may only have the gear that supports two receivers.
I had a booklet with all that info here somewhere, can't find it just now. There may be some of this info on the Dish website.