As I posted in the other thread, one other reason for the requirement for a competent person to be present in the second set is that the Call for Aid system in a Voyager DOES NOT alert the driver. I imagine it's part of PRM legislation that if the system is activated, a member of staff attends the location concerned on the train to assist as required, usually to find that mum or dad has let little Johnny press the red button by the wheelchair space, or someone in the toilet has failed the find the flush button and concluded that the red button clearly labelled "alarm" must be the one to use instead. On class 170s there is ONLY a passcom system, and this WILL sound in the driving cab. Thus the staffing agreement for multiple 170 operation is Driver in front set, SC in rear set and whichever set the passcom operates in, the person in that set resets the passcom and deals with the issue causing it to have been operated (99.99% of which involve toilets and an inability to read, based on real life experience). And no, I don't want the call for aid routed to the cab in a Voyager, not least because the trains would never get anywhere if drivers have to stop every 5 minutes and go into the train to reset a button to silence the alarm. The solution is to employ enough people, and equally value the people that you do employ. This time of year is always a challenge with families wanting to spend time together, as well as the drunken and anti-social behaviour from once a year drinkers, unsurprisingly people aren't keen to volunteer for overtime to have to deal with that and nobody should blame them.
To answer the question regarding single TM operation, if there is only one TM for a double set, there will be only one set opened up for passenger use. As to which set that is, I'm not entirely certain but I'm sure there are reasons that one particular set (front or rear) is in use.