Given the following example sentence:
The secondary effect of ability X and ability Y has been reworked.
I'm not really sure what to make of the subject-verb agreement here.
In this case, is the subject truly the singular "secondary effect"?
Or should this be read as "secondary effect of ability X and [secondary effect of] ability Y" and thus take a plural verb?
It occurs to me that the easy answer is that the sentence probably should be rewritten to be unambiguous, i.e.
The secondary effects of ability X and ability Y have been reworked.
But this makes me wonder if "secondary effect of ability X and ability Y" is even grammatical in the first place, given that the secondary effects of each ability are distinct.
tl;dr
Is the subject singular or plural?
Is the subject even grammatically correct to begin with?