It’s not a bad idea, but you’re introducing 9 new classes which need to be balanced and geared for. Maybe a good idea as a last and final update from Crate. But wouldn’t expect it for at least 3 years.
I like the concept you’re going for, but I don’t think masteries as they currently exist would fit well with it. By design they aren’t meant to be entirely self contained, and there just isn’t enough variety within any one of them to make me want to double down on a single one.
Incidentally, there was a Titan Quest mod, I think it was just called “Paths”? But it had a fairly small number of custom masteries that were dense enough that you could easily invest all your points into a single one and be just fine if you wanted to. Not far off from what you’re talking about, really.
But that’s just it, the OP is not suggesting things remain the same.
My understanding from the OP’s suggestion is selecting the same class again will open up a new skill tree with new passives and skills which are unique and fit the theme of the original class.
EG selecting demolitionist twice could open up 2 exclusive skills on the 2nd tab of the Demo mastery etc.
This would open up brand new possibilities and going “Single” class is still technically a “Dual” mastery but as mentioned before a pain in the ass to balance.