If I put a pair of telescopes in parallel, will the effect be multiplicative or additive?

This self-post was originally posted to /r/askscience. See more things from Dan's Reddit account.

Suppose I have a pair of 15x zoom telescopes (for example, I separated a pair of 15x zoom binoculars) and laid them end-to-end. Naturally there’d be some loss of field-of-vision when looking through them both. But would the resulting zoom level be 152 (i.e. 225x)? Or 15*2 (i.e. 30x)? Or something else entirely? Or am I oversimplifying?

Update: clearly I mean in series, not in parallel. With thanks to mynameistux.

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