The magic number is 50 watts per sqft. More light equals more buds and light from the top and light on all sides will grow more bud then just one light above. The problem with top only lighting is penetration to the lower branches. With top lighting only you will only get bud from the top 1/3 to 1/2 of the plant.

Yes 400w is fine for 2 plants. Use Mylar on your walls to increase the light to the lower branches it does help.