NikNak: Knowing TDM I would have no qualms about him being able to maintain competency at more than one unit. As he says, other NATS ATCOs have done it in the past (eg. small airfield in south Bucks) and SRG often allow it for 'special events' with a reduced MER.
The main problem that could conceivably arise is that due to an incident at your 'secondary' unit, your licence is pulled so you cannot work operationally for NATS at your 'main' unit.