I think your new light will be great for two plants. Probably not more than that though. I don't have any experience with LEDs though....just my thoughts from the info I've read. I've been wanting to get an LED...and probably will soon....the same brand you got so I'm interested to see how it works for you.
I wouldn't expect any drastic changes though once you install the new light. The young girls don't need much light, but your new LED will pay off when you get to flower stage.
I have grown a few Autos, one starting with 12/12 and another with 12/12 during flower. Starting it at 12/12 is a very bad idea...lol. I did it more for shits and giggles as I had some extra room in my tent while others were flowering. Anyway, 18/6 has been the best for me with autos. And I agree with you 100% that the plants do need some dark periods.