I'm growing under 2 1200 watt Bloomspect led ...

This is all you need to know the more wattage you use the more bud you will get. Just don't burn your plants up having the light too close. If your going to do a bit of growing invest in a PAR meter to measure your LED intensity to get the most out of your set up. I won't bore you with too much light science.

It wont help to only run the bloom switch. With most adjustable LED like yours if you were to choose to use a dimmed level it would be during veg, generally early, when the plants are sensitive to light and in less need of red light. When in bloom the plant needs as much light as possible so turning both on is ideal. You could dim it to save money but to increase tricomb and bud production you want all the lights turned to the max or on as much as you possibly can.

No. I am sure there is a more in depth sciency answer, but you need a mix of all spectrums during every stage of growth for the best results. The manufacturer recommends running both in Flower as well. Those lights are ~250 true watts each, if you turn off the Veg switch you probably lose ~30-40% of those watts during an extremely important time to keep the light intensity up. Also I bet you notice that there are some red spectrum lights on when you only have the Veg switch on, so if you turn those off you will lose that too. Hope my answer helps! 👊