In multiple ways:
- The chief of the First Fleet, a.k.a. the Branch Admiral has to report to the Fleet Admiral of Starfleet
- The fleets are obliged to assist eachother in missions. The treaty was later redifined, it gave the first Fleet more movement but also made it to participate in purely exploratory missions.
- Some ships and crews have found their was back and forth between the two fleets.
- Both originate from the outcome of the Romulan War
- Older starships are often passed for free from Starfleet to the First Fleet, who then repair and refit these ships.