Just leave your light at 18/6 and harvest the first one when it is ready and leave the second until it is ready. Doesn't get much simpler really.
If you prefer a more mind expanding head high, harvest when the pistils are around 90-95% brown or wait until they are 100% brown if you prefer a more sleepy and heavy body stone.
With 90-95% brown pistils, your plants will have top levels of well matured trichomes with plenty of THC.
Once all the pistils have turned brown, a lot of the THC has broken down into CBD and CBN, due to the plants metabolism having nearly stopped, and this stage with the elevated levels of CBD and CBN, is what gives you a more heavy and sleepy body stone.
So it depends on which sort of effect you prefer and the pistil colour method of judging maturity is a more true reflection of the plants health and it is the plants health that is the best indication of trichome health.
Remember too, when drying, you need to keep the flowers out of direct light, direct light on drying flowers will cause cannabinoid degradation.
Hope this helps,...........
Organoman.