If you can only have 20% other, maybe the wording should be "at least 40% of both elements."
When I read 40% of one of the two elements, I started thinking about how lots of other cards might be able to help, since, I thought, I could do something like time/aether, 6 fate eggs, 2 anubis, 4 sundials, 6 fractal/parallel universe, and tons of quantum pillars. This would be 40% of one of the elements (time).
But it would not be 40% of both.
I believe this would satisfy the rule as written (40% of one of the two elements), but not as intended (as the intent, I believe is to force at least 40% of both elements).