I think your sentence should be "Intelligence and diligence is the only way to success"
What happens if using "are" here ?
"You and John are also my friend".
That means "You are my friend" and "John is my friend".
Applying to your sentence, if using "are", we have "Intelligence is the only way to success" and "Diligence is the only way to success".
Apparently there is a contradiction here.
I think we use "is" or "are" depending on the context.(in your sentence, it depends on "the only way").
E.g: "Intelligence and diligence are good characters you need to get success".
We need teachers to explain more. I'm only an English learner.
Thank you !