I have done many of these, you have to feed both the hot and the switch leg of the circuit to the light fixture, both will go on the emg ballast and fixture ballast, one word of caution do not feed these systems from two different circuits as they are not design to have over the rated voltage on them, we just had to rewire every emg ballast in a 16 plex movie theater because the original installers wired the emg ballast to a seperate emg circuit, and the switched side to the local lighting circuit, this placed 480 volts between the switch/constant circuits, on many of the 277 volt fixtures, and cause all of these ballast to fail, and its also is against code as was said, also note that the neutral to the regular ballast is switch by the emg ballast, via a white wire with a red stripe, pay close attention to the wiring diagram, or you can end up damaging the emg ballast, if the fixtures come with prewired ballast, then you need the hot feeding the switch, the switch leg, and the neutral of that circuit to each emg fixture.
If you need a wiring diagram, I can upload one, let me know.
