To the extent that a layman cares (back to the point about signage...), I agree. But the way people seem to cope with the 7/11 pairing suggests that the numbering scheme isn't itself a problem - it's the wayfinding that really matters.Where a layman would expect to find 19 is not however "the other end of 2". What you could do is make those 1/2 and 3/4 though I suppose.
But going back to square one, and having a system that doesn't unnecessarily confuse people, the key point is to make sure that platforms 12-18 remain continuously numbered (as inconsistencies there really will confuse). And that's why I suggested keeping the same schema but start the numbering from the east facing bays so that you'd end up with them being numbered 17/18 and 19/20.