When to re-pot a sunflower?

I would carefully re-pot into a nursery container that is at least 5 gallons. Buy some good flower potting soil and add a little bit of a product called SUPERthrive, (most big box stores carry it). You would probably be better off leaving them together but if you really want to separate them lightly shake off the roots and tease them apart as much as possible. The SUPERthrive has vitamins and hormones in it which help reduce transplant shock. Also a fertilizer high in phosphorous and potassium would help your flowers really bloom.