It's integrated into the unit's systems, and these are spread across the two vehicles - so turning ETCS sets into three car units is a non-starter without ripping it out first. The ETCS does get regularly used on non-fitted lines, but this would mean training drivers on what to do with it. It also extends the time it takes to set up the cab.
It can be switched off entirely, with a conventional speedo thereby activated as a backup. However you're still leaving all of the equipment in place so no forming three cars, extra training needed for the fitters, and you've got a load of surplus complexity that's just asking to go wrong.
Better to rip it out and revert the units to standard.