How are story points agreements made?

In the past we used duration for each story, but currently, we are using story points (effort) to keep the test team effort separated from the development team effort. Story points include complexity, effort, and doubt. These three traits will differ for a developer/programmer than with someone creating manual tests or someone creating automated tests. How can all of these different factors be combined to created an overall story point estimate?

According to Scrum these should be combined together (as it's a team effort), but how is this done? Is it just a combination of the developers' and testers' efforts, or do testers and developers have to estimate the combination of efforts? Are there any guidelines for this to work effectively?