If your using cfls you should flower them soon as if you grow them really big the light cant penetrate that far down the plant so would just be a waste of time. 40-50 litre pot sounds huge! one with a ten inch diameter will be fine id say even though i use tiny ones. If yur using cfls make sure they are as close as possible without burning them- as i said before the light doesnt penetrate very far so that inch closer will make a difference. Are you using the correct spectrum cfls ? red ? and make sure you separate the two plants soon as - should never grow them in the same pots. C