With two tubes, you have more options. Plants need red and blue light to photosynthesize, but there is evidence they also use green but much less. This is why the so-called "aquarium" or "plant" tubes give that purplish hue, they are primarily red and blue. Ironically, all of them that I have so far used are alos half the intensity of the "daylight" or full spectrum tubes of the same size. I avoid these.
The best tubes have high peaks in the red, blue and green. The green balances for a colour hue that is comparable to the sun which means true rendition of fish and plant colour. A good full spectrum or daylight tube with a kelvin around 5000K-7000K usually achieves this; I prefer the Life-Glo, ZooMed Ultra Sun, or the daylight tubes mentinoed previously, all having a kelvin around 6500K.
As a second tube, you can warm or cool this, at your discretion. A lower K will be more red so warmer hue, a higher K is more blue so cooler. I personally prefer the blue added to the other. I'm not sure either wil make much difference to the plants, as they can use both red and blue so one is merely heightening whichever. As long as the one tube is the mix, this should work.
The species of plants might help decide this. Red leaf plants need more red, or more intense light in the red and green. More blue does not seem to help these plants, plus in the blue light they do look a bit off-colour, which should not be a surprise. The leaf is red to us because it is reflecting more red light, so it needs red light to be red.
Graded light is good, I do this via room lights. My tanks are all in a fish room, so when the lights come on there is daylight, and when they go off in winter there is a room light on (also timed). The effect of these moon night lights is debatable, some say they can promote algae, I don't know. I don't happen to like them.