I don't think anyone is advocating a dual hub, but merely some limited feed to TATL from SNN using spare capacity that would otherwise sit idle on the ground instead of earning money (as for your remark on the non-existence of dual hubs, Lufthansa is successfully operating a dual hub. You could also argue that SK has a dual hub at CPH/ARN). It would be particularly useful for capacity management as I suppose that O&D at SNN is much weaker in the winter than in the summer.
Btw, last year when I was looking for flights to the US, I was offered a Star Alliance conncetion via SNN with a Lufthansa flight from FRA and an onward connection on UA. Back then, I was wondering if anyone has ever done that.