term for the fact that's easier to mix 2 things if the amount are different
There exists a term in organic chemistry called miscibility which is the property of substances to mix in any and all proportions and form a homogenous substance. Now, I believe this word is a start in the right direction in our search for what seems is a highly specific and seldomly used term (if it even exists?), being it feels as it fits in the same scope.

Our instinct could now be simply to negate the word miscibility - immiscibility. But this isn't right either as it means that our substances will not mix.

I guess we could say that both of these terms are partially correct, meaning our substances are partially miscible, or better yet conditionally miscible - conditioned on the proportions used.

I realise that this isn't exactly what you were looking for, as it doesn't carry the desired connotation merely on it's own, but I believe it's as close as we can get.