Twins cant get decoupled and run as split Loco for split rake for slip & regular rake (until they arnt coupled and run as different unit)!! Its easy to attach/detach one Rake with other (as in Freight Python Rake), without disturbing loco (to minimize hassle & time)!!
To decouple Locos (and to recouple when slip gets attached), is a complicated process (carried out at Loco Shed) & usually Twins remain stable for long period and attached to train, as per availability!!