Can a controller be tasked to provide more than one ATC service at a time?
Since you've asked for a document, check ICAO Doc 4444:


  3.1.3.1 Where traffic demand varies significantly on a daily or periodic basis, facilities and procedures should be implemented to vary the number of operational sectors or working positions to meet the prevailing and anticipated demand. Applicable procedures should be contained in local instructions.


The ATS authority can indeed vary the number of working positions based on demand. The local instructions of the airport you visited will contain the details you need.