lentivirus - possible to use 2 transfer vectors? (Nov/10/2010 )

Hi,
Ok, so i've made lentivirus before, etc... but, i was wondering, has anyone used two different transfer vectors in the same virus production step?
ie, vsvg, gag/pol, rev all the same, but instead of just one shrna (or whatever), have 2 (but kept at the same concentration)?
does this sound completely stupid?

V

Of course you can do that, but why not generate the lentiviruses separately, titer them and then mix them with the equal titer?