Savoy cabbage plants producing two heads on the one plant. The second head is growing out the side at the base . Will this stunt the growth of the main head? The plants are set about ten weeks.
One thing about plants -- they don't read rule books. They often do unexpected things, and this would be one of those that lies within the Brassica (the genus to which they belong) repertoire. Think brussel sprouts. So I would let them both grow for awhile, then cut off the bigger one, though if they sprouted at the same time, they're probably both going to be ready for harvest at the same time. Or you could cut off the smaller one and eat it now, and let the bigger one grow. Or leave them both to grow for awhile. Kind of up to you.