If you select a Cruiser Factory, then Frigate Factories, then queuing up Cruisers will spread the Cruiser construction onto the Frigate Factories as well.
This does not appear to be the case in the opposite direction or with other combinations (cannot trick Cruiser factories or Advanced factories to build Frigates etc). This is likely due to lingering code from previous builds which the Frigate Factory was used for Cruiser production.
