While you’re logged into CityStrides, go into your Settings (linked from the top right menu), go into the Activity Trackers section, and click the Connect button for the other service. That will (outside of Garmin) automatically sync in your history from that service & it’ll skip existing activities so you won’t end up with duplicates.
After connecting a second service, you may want to pause one of the services if you’re actively tracking with both of them - that will decrease the load on CityStrides.