For holding paintbrushes and/ or coloured pencils, e.g. aquarelle etc. in an art studio.

The holes for the brushes/ pencils are 7.5mm in diameter in the base and those in the upper shelves are 9mm. There is a front tray (Width: 215mm x Front-to-back: 45mm x Depth: 12mm) for odds & ends such as erasers, pencil stubs or other small artist's tools etc.

There are a couple of configuration options here, with a larger base with a tray in front, a smaller tray without, as well as two heights of pillars, where the shorter pencils are held in the front and the usually longer brushes held in the higher rack to the rear.

If both of the sections are to be joined as a single unit, the two assemblies can be glued together with cyanoacrylate/ superglue (CA) glue, being aligned and mated together using the two key slots at the flat, rear faces of each base section. (Since CA glue is the permanent option, there is an alternative, such as E6000 which gives a fairly strong bond but can be loosened ~IF~ the glueing is done neatly in the first place and the debonding is also done extremely carefully, should it prove necessary.)

I would suggest the two assemblies are placed flat with their rear faces towards each other and the two 'keys' (small rectangular pieces) being neatly pre-glued (and the glue cured!), one key into each slot in ~one~ of the two rear faces.
The opposite face to that with the keys in, is then coated with a thin bead/layer of CA glue, which must not touch the bottom corner of the face as it rests on the flat surface (otherwise the glued surface will likely stick to the flat surface being used for assembly, a make quite a mess!).

These two sections are then drawn together, bringing the mating faces into firm contact with each other.