DoN. Nichols wrote:

Be sure to order a matched length set of belts, or one will be
too lose and not transmit much power, putting all the load on the other
belt.

The Gates people decided about 10 years ago that there was no more need
for matching belts. They contend their current manufacturing process is
such that all belts for a given part number are within the same
tolerance as any two matched belts.
They once shipped belts in matched pairs taped together, and marked
the catalogs with an asterisk for those applications requiring matched
sets. Now no Gates belts come matched, and the catalog no longer lists
applications as requiring matched belts. They just show Qty 2.
However, every belt carries a lot number next to the part number on
the belt. The lot means all the belts were cut from the same sleeve. I
would try to find two from the same lot number if I were buying for
myself. If I were the guy behind the counter selling you belts, I'd
recount the above story and save myself some effort
And by the way, most small parts stores will not have more than one
each of these A- and B-series belts. They may want to substitute
fractional HP belts (all-numeric...2270, 3310). Those would probably be
OK, but the likelihood of that store having two from the same
batch/sleeve is pretty low. You would have better luck at a bearing
supply house or a big independent auto supply. Stay away from the big
national retail chains, they will waste your time on something like this.

Other brands may still have matched belts, but I haven't seen any other
brands in a long time.