The flower spectrum is a booster to the veg spectrum, not a spectrum to be used on its own, so yes, use both the veg and flower spectrums together.
To get your photo plant to flower, you will have to go to 12/12 however.
It is best to never mix photos and autos together, photos won't flower at 18/6 and autos won't yield much at 12/12.
I am also very against growing any sort of cannabis at 24/0, plants NEED sleep and 18/6 is much healthier for photos in veg or autos for the full cycle.
All plants need a dark period every 24 hour period to finish their photosynthesis cycle and have a correctly functioning metabolism.
A plant with an incorrectly functioning metabolism will never grow to its full potential.
The MYTH that it is ok to grow cannabis for weeks/months without sleep (24/0), needs to be removed from history asap!