That sounds like a good way to stress out two plants to me.maybe switch it for a week or so cause if the auto is a true auto it will continue to flower when you switch the lights back to 18 and your photo should revert to veg
Who in the world told you if the photo has not reached maturity it will veg while under 12/12 until mature? Thats the biggest crock i ever heard lol...That sounds like a good way to stress out two plants to me.
I'd wait till I switched the photo to 12/12. It probably doesn't have the auto flower trait locked down in the seed, either the strain wasn't worked properly or you have a rare case. What strain are you using?
Or if it's getting way to big you could switch to 12/12 now and the if the photo hasn't shown preflowers it will keep vegg'ing until it reaches maturity.