You could take it apart and see how it's wired. If each set of two bulbs are wired in series, basically then say each bulb is 20w, then it's shooting 40w through the whole system. Remove tw of the caps go end to end it would end up at 40w (numbers are just an example). It just depends on how it's wired. If it's in parallel, it probably won't work, if it's series, it should.
Should is the key word. lol.
So if I have 1 ballast per 2 bulbs I should be fine running 1 48" bulb off each ballast?