This is for a vehicle I assume? One lead is the Satellite (usually yellow if I remember correctly) and the other green lead is Terrestrial for ground based repeaters. As long as you are out in mostly open area and not in somewhere like NYC with big buildings you should be fine with only the Satellite lead.

They do sell adapters as well but not necessary. But if your antenna is that old that it has both leads its probably a big ugly one- you'd be better off replacing it with a newer micro style antenna. Unless its like an OEM antenna thats thru the roof- even in that case i'd just leave the old one up there and use a new micro antenna. Some of those old ones weren't the greatest.