I have had a doctor recommend wearing one insole on two different occasions.
Once when I had osteonecrosis in my left foot, I had a very stiff insole that was meant to keep the affected foot from flexing too much. This was a prescription thing shaped to fit my foot made from carbon fiber or some such.
The other time I was wearing a walking boot (for the osteonecrosis, which had previously been misdiagnosed as a broken bone). The walking boot had my left foot higher than my right and my lower back was completely jacked up. Doctor suggested I buy inserts and wear one in my right shoe. I got them at Foot Locker, can’t remember the brand, but you can get them anywhere. Helped a ton.
Re-reading the OP, I will add that the lower back pain when I was wearing the walking boot was on left side, same side as the boot. The insole was in the right shoe, so yes, the opposite side from the pain.
You need to ask you doctor questions. Go with a list to help remember. Or a friend/family member. And doctors aren’t always right. Insurance should offer you the ability to get a second opinion.
If you’re off balance, wearing one insole will help balance you out; why wouldn’t you trust him & try it out?
No, don’t do this, go with an Even-Up; saved my [del]ass[/del] back when I was in a boot. Two caveats, though:
[li]It was much cheaper on Amazon (& I’m not a big Amazon fan; don’t have prime.)[/li][li]I had to jury-rig it by buying a second velcro strap to wear across the top of my foot (behind where the shoe ties) to keep from accidentally kicking it off like a shower flip flop.[/li][/ul]