One way is to use a Quadcopter or crossbow get a thin line between the two seasteads, then use that to pull across a thicker line, and then use that to pull across a cable. The line/rope/cable could be left connected so a winch could pull everything over quickly. After the cable is secured to both seasteads the sea anchors could be deployed and the kites for the two seasteads could pull to different sides so that the cable between the seasteads was pulled tight. Then a cart could move back and forth on the cable between the seasteads carrying cargo and/or people.
There are thrusters that can get 760 lbs thrust from 6 hp. A seastead might have several of these. So thrusters could be plenty to provide tension on the line.
The oil industry has an extendable walkway that can go between a ship and a platform.