use the DLI chart i have in my profile comments and figure out if it is too much... the plant will also fall over at that stage if "too much". but, you might be able to run for 16 hours and reach a 'max' DLI for your context and save some money while providing the different spectrums of light and the benefits that brings.
if you have umol/s spec, it's easy. add up total umol/s from both lights, divide by sq ft. multiply that by 10.7639 and that is your PPFD value. Use that value in the DLI chart and hours you run the lights..
with co2 -- 18 hours -- 1000 ppfd is up near max.. and you don't want to reach max here because you aren't doing everything optimally -- i dont say that to be mean.. to get this high you need to be dialed in in all ways possible .. laboratory type precision.
without co2 18 hours -- 667 ppfd -- not as crazy as the max with co2, but similar concepts.. could probably shoot a bit higher than this depending on how high your lights need to be.. hotter lights need to be stronger at a higher height. more lost to absorption etc..
12 hours -- same strictness above, more so with co2.
with - 1500
without - 1000
you can see those ar ebasically 43DLI / 65DLI at those lengths of time... don't go over that .. run at X hours? where do you hit 43 or 65? that's as long as you want to go. if it's less than ~15 hours, logic dictates that you shouldn't have both on, for example, or it might start to flower, or you run longer and hurt your plants with too much light.