Matching belts for fan/AC?

If one belt is loose, in a pair of side-by-side belts, it will squeal under hi revving. You can over tighten the belts so the shorter belt is over-tensioned a little while the longer belt just attains the correct tension.

The next best thing if you can't get matched belts is buying two from the same manufactured batch lot. If you buy them from a high quantity seller, you stand the best chance of this as they have the highest turnover, so they should get larger batch stocks.