You are not able to dock to more than one entity at a time in the current DEV build.
The old ship's core docking beam might be retained for use as an activation beam, it can currently be used to toggle activation blocks.
If I've been reading the posts correctly the rail docker docking beam is just temporary, the plan is to have the blocks connect magnetically when brought together close enough and in the right orientation. Using the proposed USD or even Sven's variation on it might cause problems when that happens because both sets line up.
Hopefully this means the ships are mutually docked, each ships systems all work, and if one decides to move, both move together. To get both ships thrusters working both ships would need pilots who coordinate their thrust. Otherwise it would be like one ship towing the other.
I imagine there are going to be several competing designs for a functional "USD", which one wins the "USD Wars" is yet to be determined.
To accommodate his door activation logic, Sven simply moved the docking blocks down one row from where mindlord has them, and both are missing the camera at the top middle block that faces outwards that my design has. Chances are real good the design adopted by the majority of players has yet to be designed and/or shared with others at this point.