It depends on the type of LNB. There are at least three LNBs that match the description you just gave. If you had the latest generation, a Dishpro Plus Twin for the D500 dish, you'd only need one wire from the LNB assembly into your house, and then it'd go into the separator (looks like a splitter, but it isn't) into the two SAT inputs of your receiver.
As I said, that's not enough information. That could be a DishPro Twin, A DishPro Plus Twin, or a Dish Quad LNB for all you've said. Each of those models has slightly different wiring requirements.
Look at the sticker on the LNB I linked. Does yours have the same sticker? If that's the sticker you have, then it only takes one cable and a separator. If yours doesn't have the same sticker, buy one that has the right sticker; it makes life easier.
Cowboy I think he means he has 2 single LNBs on his dish seeing he says there is a space between them. Most likely they are LEGACY and probably singles rather than duals. Best in that case is replace with a Dish Pro Plus TWIN.
LEGACY single or dual have plain grey plastic.
Dish Pro single or duals have black DP logos on the back of the LNB and could be easily connected to a DP34 switch.