Is it better to have one light unit that covers th...

Id put both sets up check your light output more light the better. You want your leaves to get lots eh couldnt ya if yad a dwarf that neede more you could add aclittle one or even run some on the side. Get that tentbuddy app you can measure your light with phone or tablet

There are advantages and disadvantages to both, I'm sure... but with 2 lights, you can have a couple of plants in the flowering stage and a couple in veg state with the lights hung at the proper distance for both... I've never dimmed my lights although I have the ability to do that... I just change the height of them if I need them a little less intense. The way I think about that is to consider the sun - the sun sends out the full spectrum of light every day and whether you've got a young tomato plant just starting or an old-as-the-hill grape vine, they get the same amount of light every day. But this is where two lights can come in handy - lowering the lights to "feed" a smaller plant (with the same spectrum) while raising the other one to "feed" a flowering monster you're about to harvest seems quite worthwhile. But again, like your question on the tents, this is going to come down to what you can afford and how you want to approach your grows... Good luck!