Dear Matty,
You can certainly have the 0-12VDC old style and the new Live Steam 17VDC controllers for one line.You could use the Hornby power track and set up a switch to select either transformer/controller.Im sure Maplin would do a rotary switch to do this.The problem comes when using your loco's on the line.If you accidentaly put a 12vdc train on the track when the live steam is selected,if will whiz off,crash or have the motor burn out as live steam is constantly live at 17VDC.You could fit warning lights which are also operated by the switch to aid you,but you are still taking a big risk and mistakes are easilly made!!
All the best,Stew.