Larry Pletcher's tests (blackpowdermag.com) of priming powder timing indicate that priming with 2Fg averaged 70 msec, compared to 40 msec for NullB, 60 msec for 3Fg. He also notes that there was more variation in ignition times with 3F and 2F as prime. 2Fg in the pan will result in ignition, but it will be a little bit slower than other choices available to you if you prime from a separate horn or charger.
So, it'll work, but not quite as well. You might not see the difference hunting or on a woodswalk, but probably would give up a point or two in a 20 shot aggregate if shooting paper bullseyes.