I want to use two different endpoints of Twitter API V2, side by side.
Endpoint 1. GET /2/tweets/counts/all
Endpoint 2. [GET /2/tweets/search/all]
When I looked up the rate limit for each of the APIs, it is 300 requests / 15 minute period. Does that mean I can make 300 requests for endpoint 1 and 300 requests for endpoint 2 separately, or altogether of 300 requests per second for both of the endpoints?

The rate limits are separate, so you can use them at the same time, it’s 300 / 15 min separately.