Prismatic omen and gruul turf
Gruul Turf will have 6 separate activated abilities. One of them is to tap for RG, the others each tap for 1 mana, 1 ability for each color. So no, you cannot tap for 2 mana of any color. 

Also a note on terminology, tapping a land adds a mana to your pool, it does not add a land or land type.