I'm pretty certain that namida's challenge can be extended to 27 combinations of two skills, likely even all 28.
In fact, I think that's an idea for follow-up challenge; but right now we're in over-supply of challenges, so I guess I'll leave it for later (and have the time to actually set up a level for it, though I think it's easier than my current challenge).
I guess as soon as someone calls open-floor, we're back to one challenge again.
Though actually, it doesn't seem to harm having two challenges running simultaneously...