I would go with 2) 6500K. Looks more natural, 5000K is too yellow for my taste. With 2 x 6500K you will still have 6500K as this is color temp.
is it ok to use DIY co2 in this lightning or should i reduce the lightning?Kelvin refers to the color spectrum, has nothing to do with power. They dont add up as you suggested.
Both are very good plant spectrums, beyond that is just a matter of which one you like looking at.
Personally I'd do one of each