Hi,Bifter,its not a god idea to put your seedlings under a hps lights,and certainly not under two of them.Hps lights are for flowering not vegging,you need MH (Metal Halide) lights for seedlings and vegging,or CFL's.You see the seedlings require the blue spectrum from MH,or CFL's,your hps lights are red spectrum for flowering.They might grow under that light but not very well.If you are going to use that light anyway make sure it is quite a distance away from your plants or they will burn,at least 3 feet away,and be prepared to see your seedlings stretch looking for that blue light spectrum that your depriving them of.
Good luck.