Sizes are like a diamond or interlinking diamonds. There are two lengths at two ends and two widths at the other two ends. There are four combinations available. Two will work but will be poor in comparison to the other two. If there is a length and width difference, you may want to fit the longer length and narrower width to balance both feet.
Some customers require mismates.
A size and a half difference is the most you can fudge fit and get any kind of comfort. If there is a drastic width difference also, then a full size or less difference may force you to mismate. Mismates can be done in two ways. This allows you to choose any shoe pattern made. However, even with the discount, this can be expensive.