combining 2 different lighting types

I have been growing indoors for about year. This is my 3rd crop and I was wondering if combining two separate light sources if negatively affected my crop. My main light source is a 600 w. HPS light and when the plants (4 at a time) get real tall I place another light shining down toward the middle of the plant. This light source is from a 60 w. florescent bulb. Do the spectrums mix and affect my plants negatively? are some spectrums cancelled out?

Just wondering....thx

I would say no.
I've never used the Dual spectrum HID Buld
but I've heard good things about it.

I've heard of people adding a 6500K CFL with there HPS
all the way through the grow.