I can't remember which thread it was, but I remember hearing a lot of good bros saying it was a bad idea. Their reasons were along the lines of: "it raises the chances for an infection". Which I don't quite understand myself. If both substances are sterile, then how can mixing them raise the risk for infection. Maybe it has something to do with the idea that some of the water-based solution will get encapsulated by some of the oil-based solution.
However, I am constantly hearing people claim that adding little b12 (water-based) w/ juice that is painful (prop, t400, etc) reduces the pain.
Anyway, 2 injections per day wouldn't be bad if you know how to rotate injection sites. I personally only hit the quads...but I am planning on trying out the delts and glutes this cycle.