Re vegetating during flower?

The dark green, burnt tips and the huge contrast vs "healthy" new leaves, shows kinda why I say there's nitro excess. Those new leaves, are simply healthier, notice their colour.

Nutrient excesses, too much nitro, hormonal dysfunction...probably exacerbated by the extra light (like the plant thinking the light is increasing, as if it's outdoors). The hormonal dysfunction and nitro excess, along with that lighting encourages vegetative growth, so the plant is like "hey, I can grow for longer". As a result, you get that foxtailing.

I agree with LostworldGenetics, the spectrum of the two plants is totally different, this gives all kind of revegging problems. Especcially if your blurlpe was old. @styaliaga had s similar thing going on changing from blurple and I have a revegged monstergrow going on because of this.

I would say bad/unstable genetics. My solution would be to return to using only one light. No doubt the extra lumens has signalled your plant to go back to veg due to the extra photosynthesis that is occurring now you have doubled your lights, almost as if your plant has been transported from Fall, back to the middle of Summer. In future, whatever is happening at the beginning of flowering, should be maintained all the way through to harvest and to not mess with anything that will cause major differences to the way the plant is flowering. You may now have a very "confused" plant and whether it continues flowering or keeps revegging is difficult to say. Maybe going to 12/12 for a week might stop the revegging, but unfortunately, I think your plant could be too confused to do either option properly. Hope this helps, Organoman.