I think maybe it's a bug, or if not then you've taken it too far. I'm fighting a mob and he is not immune to my primary wield. I do something with the primary wield, but because the mob turns out to be immune to the dual wield weapon, I get the message that I can't learn anything. Here it is.

<559hp 312m 787mv 16687tnl (-8.36%)> whirl
You whirl an unholy axe, 'Nightbringer' impressively, but fail to hit Orcus, the Prince of Demons.
You can't learn the nuances of whirl since Orcus, the Prince of Demons is immune to your attack!
Your whirling axe misses Orcus, the Prince of Demons.
Orcus, the Prince of Demons is in perfect health.

<559hp 312m 787mv 16687tnl (-8.36%)>
Orcus, the Prince of Demons's cold touch drains some of your life away!
Orcus, the Prince of Demons's deathly touch MUTILATES you!
Orcus, the Prince of Demons meets your wrath with a resounding impact.
You deftly duck under Orcus, the Prince of Demons's claw.
You dodge Orcus, the Prince of Demons's claw.
Orcus, the Prince of Demons's claw EVISCERATES you!
Orcus, the Prince of Demons is in perfect health.

<476hp 312m 787mv 16687tnl (-8.36%)> flee
flee
flee

Your wrath MASSACRES Orcus, the Prince of Demons!
Orcus, the Prince of Demons is unaffected by your black light!
Your wrath DISMEMBERS Orcus, the Prince of Demons!
Orcus, the Prince of Demons is unaffected by your black light!
You dodge Orcus, the Prince of Demons's claw.
Orcus, the Prince of Demons's claw EVISCERATES you!
Orcus, the Prince of Demons has a few scratches.

He is not immune to my attack. And it wasn't about learning anything of the offhand weapon. It was the primary weapon, which did affect him. Bug?

The learning code isn't sophisticated enough to know which hand is wielding a useless weapon vs. which if either or both is being used to execute the skill you're trying to learn.

Ergo, by design.