Note that the Netherlands is a bit of a special case.
Below FL245, Air Traffic Control is provided by Luchtverkeersleiding Nederland (LNVL, the Dutch Air Traffic Control) and Dutch Mil (Dutch military ATC). Until one year ago, the Dutch military operated from their dedicated air traffic control centre in Nieuw Milligen. Since January 2018, they are colocated with LVNL at Amsterdam Airport Schiphol and largely use the same systems as LVNL.
On the lower area map (below FL195), all sectors marked as Amsterdam Radar are controlled by LVNL. The sectors marked as Dutch Mil are controlled by the military, and despite some of the sectors are still named NW Milligen, they are controlled from the same room as Amsterdam Radar at Schiphol. As you can see on the upper area map, all sectors are marked Amsterdam Radar.
Their systems are partly combined, but military sectors and civil sectors are never combined. So if a lower sector is controlled by Dutch Mil, the upper sector will be controlled by a different controller.
When both the lower and the upper sector are controlled by LVNL, they can be combined during hours of low traffic. In that case there may be a single controlled controlling flights in the lower airspace and in the upper airspace. Combination is possibly also with adjacent sectors.
But wait, that's not all. Note that the map mentions that the Upper Area is between FL195 and FL245. The upper airspace mentioned so far in this answer, has a ceiling of merely 24500 feet.
Above FL245, ATC is provided by EUROCONTROL's Maastricht Upper Area Control (MUAC). They are located in Maastricht, far away from Amsterdam. They use the same radars, but their control system is totally different. Sectors of Amsterdam and Maastricht can thus not be combined.
Within MUAC's airspace, there is horizontal and vertical sector splitting. Especially the Delta sector is a busy one, with lots of transit traffic to/from UK/Atlantic and with climbing/descending traffic from/to London, Frankfurt, Cologne/Bonn, Brussels, Amsterdam and many more airports.