Subject-verb agreement with more than one gerund in the subject
Solution 1:
It depends entirely on context and whether or not you are using the two things individually or referring to a single collective thing.
The sentence you provided is ambiguous but you can make the meaning clear (either way) by slightly rephrasing it.
For instance:
Both reading and studying history have been formative.
The activity of reading and studying history has been formative.
Without rephrasing it, an argument could be made for either the singular or the plural. It depends on what you're trying to express. Neither would necessarily be wrong.