The system you are questioning is one we use at Cardiff. Due staffing we are only able at the moment to have 2 radar controllers available between the hours of 9-5. However we always want the aircraft to communicate on the correct frequency , so no hastle for either the pilots or ATCOs over which frequency to select Dependant on the time of day.
Outside of these hours we "bandbox" 3 frequencies together during the period 9-5 however two controllers would be handling 3 frequencies.
Safety wise it has many advantages, sometimes again due staffing we are unable to split the positions permanently during the day but again the aircraft are on the correct frequency so if there is a necessity or ability to split back to two positions it only takes the pressing of one button and the ATCO has the positions split, which means you are not instructed to change frequency.
It may seem daft but it is a good ,safe and quick system for us to use.