DON'T GROW MULTIPLE PLANTS IN THE SAME POT! The roots will become intertangled, and you'll end up losing both plants. Get them into their own pots as quickly as possible, preferably from the start. You can use plastic beer cups with holes cut or drilled in them for drainage. You'll have to replant once or twice, but that's normal...and it's better than losing plants. Plants that are 3-4" apart in a pot will have their roots tangled within 2 weeks, probably sooner.

You really need to wait until the plant is at or near sexual maturity to switch over to flower. Check for when your plant has alternating branches/nodes; that;s usually the sign. Keep your lights as low as you can to prevent stretching. LST the plants if they're going to be too tall; expect them to double or even triple in height in flower. This thread

http://boards.cannabis.com/indoor-gr...-quantity.html

asks some of the same questions; search around for more answers. In my experience...figure 3 weeks minimum until the plants is ready to be switched over to 12/12. And that will be a pretty small plant. The difference between 3 weeks and 5 weeks is huge.