I use both brakes all the time, unless I'm in the middle of a turn. I trail brake a lot, even on the street. Whatever you do, do not try to trailbrake into a turn.
You still need to hone your feel of the bike. Trail braking is a bit more advanced.
You just have to get the feel for the lever. After a while you don't even think about it anymore, and you just know (for the most part) how much brake to give it. I normally use both brakes unless, like I said, I'm trail braking.