In common usage there may be little or no difference in meaning; but in mathematical writing, divisible has an accepted usage not held by dividable. Oxford Dictionary gives “Mathematics (of a number) containing another number a number of times without a remainder: 24 is divisible by 4” as sense 2 of divisible. Sometimes redundant wording like evenly divisible is used, and I suppose evenly dividable could be used, but it isn't.
I don't know of a general rule. Incidentally, as seen via links in google books, dividable is a word respectable enough to use, in spite of suffering some disrespect, as in following:
