Well not exactly the same result,,for a few reasons.
One would be that the 400watt would need to be a bit closer to the plants to give the best possible 'brightness' (lumens we shall say) to the canopy/plants. Another would be that the 400 is losing a small percentage of its output by being upside down. As in the fitting doesn't emit light. Etc.
I'm assuming you have the 400watt on a stick/pole and the 600 dangling yeah?
With what you have I would either ditch the two and go for a single 1000, or after this grow, switch to having the 400 on one side and the 600 on the other, setup a small veg/clone space with a cheap cfl and then get a perpetual grow on the go. As the plants go into the flower space they go 400 then move over to the 600. We can dream though aye!