You can do as you want, but it's more optimal to go by the color of trichomes. Even check them 2-3 days after you chop it down. See if it developes further - it typically does. Might impact future choices.
too long could cause it to die on the plant.. .don't want that.. .so just keep eye out for overt problems. You want to cut at same time? then, pick out a day the triches on one or both are close to what you want. one or both is likely to be suboptimal if you do it at the same time, except in instances of coincidence. so, i think you've answered your own question.. .you are going to harvest them at the same time, so pick a time at which both are decently ripe.