Got back-to-back wall 110vac outlets. Let's call 'em outlets 1A/1B, and
2A/2B.

Presumably, these are fed by a common wire.

1A/1B have been in daily use for several years. Don't know when 2A or 2B
were last used.

Yesterday, 1A/1B, and 2A were found dead. How can this be? I thought if 2A
were dead, 2B would have to be dead also. But, it's still hot.

How could 1A/1B be dead, with 2A dead?

I'm fixing to troubleshoot, and need to know what to look for (yes, I have
the little neon glow testers and a multimeter). I'd like to have some idea
of what to expect. Right now I have none.