I am attempting to write an endpoint in Python Flask that requires inputs from 2 users to run the function. I would like to have it so that user 1 would send a request with inputs to the backend and then wait for user 2 to send inputs as well. The endpoint would then calculate a result and output it to both users. What is the most efficient way to do this?
This is best done by some asynchronous mechanism. The best solution will depend on your exact use-case.

Webhook - If user 1 and user 2 are other applications using your api, the simplest way would be with a webhook mechanism. Where user 1 and user 2 would subscribe to the api by depositing a url that your application calls with the results once both inputs are sent.
Polling - You provide an endpoint that both users need to poll to check if the api is ready to send back the results.
Email - You simply email both users the results once you receive both inputs. Or SMS, or IM message ...
Persistent connections - With a mechanism like websockets or http2 push. You can achieve this with a python application. But it this is the most complex solution and in most cases not needed.
Multiple Users Connecting to an API Endpoint