Second cabbage crop advice please

Hi, I started my cabbages in mid April and have read if I make a cross in the stump after harvesting I will get a second mini crop - is this mythical gardening witchcraft or true?? I can’t wait to try but two questions - my two cabbages (yes two survived 😰 hence the desire to maximise) look readyish to me but I’ve read 20 weeks from planting, is that about right, so another 4-6 weeks? I’m a cabbage virgin. Also, some sites say a shallow cross, others a deep one, anyone tried both and can advise which works best? Many thanks in advance!