MH and HPS together?

If MH promotes good veg growth and HPS is used for flowering, would there be any benefits derived during the flowering stage from using both types of lights simultaneously? The set up in question uses both 400W and I'm going into the 2nd week of flowering on one plant and the first week on another.

yes yes and yes

Another question; will my plants fill out more and grow in height less during flowering? In the past 2 weeks of flowering one of my plants almost doubled in size to 3' and I guarantee it isn't from lack of light. I was sold g13xbuddha's sister and the other plants aren't nearly as tall as this one.

plants will 2x even 3x in size when flowering each strain is different but all will increase a fair amount in flower.

the more light the better they will love the extra MH, i flower under both and almost doubled my yield, then just 1 HPS.
expect your plants to atleast double before there finished. good luck

Thanks everyone for your feedback - makes me feel better about what to expect. Fingers crossed that everything goes well leading up to harvest.