This may have been answered before, but I can't find it anywhere and am too afraid to test it myself.

If I combine multiple units with the same skill do the skill up chances stack or is it a waste of resources?

If so, how is it worked out? Eg. to get to 4/5 one unit is 25% chance, would two units be 50%, 37.5%, 31.25% or simply 25%?

Basically, I'm trying to max Belinda while the event is running and want to know if I can do it more efficiently.