I'm thinking about trying to run two 400 watt mh bulbs in my projector to see if it increases the lumens. Has anyone tried this and would it be possible to run two bulbs with one ballast?

Maybe you a are ready to try the Frankenfresnel?
http://www.lumenlab.com/forums/index.php?showtopic=8219

Seems interesting, but I just wanted to add other lamp (that I already have) to my pj. Just for a quick experiment. So I needed to know if I can run 2 lamps with one ballast. Maybe if your experimenting turns out well I may give that a try.

The ballasts are designed to drive specific load (i.e. 400W) it will not be able to drive two lamps you would need a second ballast. There will be some increase with the second bulb but its probably only 10-20% not 100%. The light needs to be columated to pass through the LCD so you need to block light from one bulb that is going to the other fresnel so you get two brighter spots but you loose the contribution from the other bulb that would hit that location, not worth the effort and getting rid of 400W is hard enough 800W would really be a problem.