They have refueling systems.
There's no orders active.
After some messing around with joining detaching I'm now able to get one tanker to refuel another.
It does however transfer all of its fuel into another tanker, so that doesn't solve anything
In the old VB version there was the option to equalize fuel between, that would be handy now.
I think I can solve it, by constantly detaching ships to create tw-ship fleets, where the fuller one transfers its fuel to the other, keeping the minimum.
Then create a new fleet with the 'full' one and a new empty one. Repeat.
edit: I can also transfer out a ship that's being refueled before the refueling process finishes.
This much micro management can't be right.