@meathead: do you mean that probably they are the same/similar part, but only with 2 different part numbers?
If so, would buying 2x of a single (either 'a' or 'b') would be enough?
Yes, I believe so. Although I suspect one number might be discontinued. If so, you might be delayed in getting them. If you are set on OEM, I'd cross check with another parts company and see what they list.