I think you're confused, that is exactly what Synergy can mean. It means the abilities of the two classes can benefit each other. How is 2 classes with abilities that match perfectly and synchronized cast times NOT a benefit?

syn·er·gy
/?s?n?rd?i/ Pronunciation Key - Show Spelled Pronunciation[sin-er-jee] Pronunciation Key - Show IPA Pronunciation –noun, plural -gies.
1.combined action or functioning; synergism.
2.the cooperative action of two or more muscles, nerves, or the like.
3.the cooperative action of two or more stimuli or drugs.

I'm pretty sure that fits #1 perfectly. In fact it fits it better than how you're trying to define it (the damage bonuses of the priest benefitting the locks).