Can I connect two services at the same time, e.g. Polar and Strava?

Yes, that works. The rule is simple:

• One connection (e.g. just Garmin or just Strava) is free.
• A second simultaneous connection (e.g. Polar and Strava) is Pro.

A second connection makes sense, for example, if you record with different devices or can only get your runs into Trait via a second service.

Watch out for duplicates: If the same run exists in both services (for instance because your watch also uploads to Strava), it arrives in Trait twice. In that case, delete one of the two activities – more on that in Why is an activity shown twice?

You set up connections under Home screen → Settings → Connections (see How do I connect my watch or Strava to Trait?).