I used to play all kinds of games with stacking props on .049's. Its not hard, and works amazingly well. One very successful stacked set was 2 of the 3-blade props simulating 6 blades, with just 3/4 inch clipped from each blade. (I think those were 5 inch dia props... the stock Cox 3-blade from the mid 1970's, they only had one.)

There's no keying or anything really needed. Just rough the two prop hubs a bit with sandpaper and put them on a long screw shaft. They stick n alignment quite well even using the Cox Spring starter or an R/C car motor converted to act as a 1/2 A starter.

If it were only that easy to make up a home-brew fan for a .40.... but the shafts aren't long enough.