1985 300D Alternator - double pulley running on one belt?

[BlueGermanTank]

Hey all, first time poster.
I recently changed out my alternator and all my belts in the process. Unfortunately since I haven't been able to find a matching pair to replace the ones that go on the alternator double pulley. What I'm wondering is, it's a 65A alternator, and a small water pump. That doesn't seem like a very high load to warrant two belts, except for redundancy.

Is there any reason I can't run the alternator on a single belt? It keeps throwing one of the two, but the other is on snug and the car still runs. I see some old posts saying that it's better to have both, but others saying you can't buy "matched belts" from suppliers anymore.

Wouldn't it be better to run one belt, rather than overtighten two to compensate for slightly different sizes? I have to imagine the bearings would last longer.
Thanks in advance!