Best time to switch from MH to HPS in flower?

I have a 400 and a 600 in cooltubes stacked running vertically. I only have a hps bulb in the 600, but have both mh and hps for the 400. So, my question is, what's the best time to switch the 400 mh to the 400 hps in flower? I'm thinking maybe run the mh bulb for the 1st 2 weeks in flower, and then switch it to run all hps bulbs for the middle 4 weeks, then switch back to the mh bulb for the last 2 wks during flush to help them crystal up. Any thoughts?

ps. Keep in mind that the 600 is going to be running an hps bulb the whole grow either way. I'm just concerned if the switching out the mh in the 400 at specific times would be beneficial.