Hi håkon295,
Welcome to Apple Support Communities. It sounds like you want to know if you can start a workout in the Runkeeper app and also use the Workout app on Apple Watch simultaneously. We can see why you'd reach out about this; we'd like to help. It's only possible to run one workout app at a time on Apple Watch, however the Activity app should still recognize the movement toward your Move and Exercise goals.
You can set up the Runkeeper app as a source for your Health app to ensure the data is tracked there as well; take a look at the section titled "How Health handles data from multiple sources" here: Manage Health data on your iPhone, iPod touch, or Apple Watch
Best Regards.