lights

Whenever you want won't hurt in the slightest if you keep temps under control.

if not overpowered, the 50w will probably outweight any other factor... and it's a flower spec? so, probably higher red ratio? That also is better for flower. But even if the other light has more "blue", the extra 50% watts will outweight that factor.. assuming you "need" more light... if already providing max DLI relative to your environment, adding more is only going to hurt the plant... if after a few days or even a week they start getting all droopy-dog the last 1-2 hours, you're probably giving too much light, assuming something more obvious doesn't occur like some burns on closest leaves or wilting for no other apparent reason... bleached buds in flower... super tight node spacing... lots of ways it can present