Both front wheels are driven through a front differential - it cannot be locked. As long as they have equal traction, they will pull equally. If one has no traction, it will spin free, and the other will not drive. In this case, your two options are to restore traction to the free front wheel, and/or, apply the rear differential lock, so that the rear wheels alone get you moving. The front wheel differential cannot practically have a lock, as it would tear itself up in tight corners with good front wheel traction.