Surely it's a lot easier and less expensive to build an engineless, ship "trailer" than a self-propelled ship with an engine, fuel tanks, propellers, a generator etc?

The ships are already at the maximum efficiency size given their engines, so to pull a similar sized ship they’d have to add an engine to the first one, in which case why not add it to the second and have two ships?

Could they be pulled by multiple small tugs instead? (no idea if that's feasible on the open seas!)