I can personally attest to the success of putting two props on one shaft. I saw this done on a "ducted fan" home-built built by an older club member. Nobody doubts his methods because that distinguished gentleman somehow makes the seemingly impossible work just great!
Two, three blade props stacked on the shaft would definately improve the rotational inertia but the thrust?... try and see....

The funny thing about trying something new is the amount of people denouncing it before the testing and the amount of people praising it after it works!

I'll do some loading tests...