I'm baffled by the notion that the plunger and the toilet brush are in any sense a pair that need to be placed together, like say salt and pepper grinders. They absolutely aren't.
The toilet brush is something that anyone who uses the toilet, whether resident or guest, may need in order to leave the bowl as they found it; which I think we're all agreed is a basic social duty. So it needs to be placed close to the toilet in plain sight; no guest should be obliged to hunt through cupboards or ask their hosts for it, both of which are embarrassing.
The plunger is a tool that would only be required if the toilet should malfunction. In most households this is thankfully a rare occurrence, so that it no more needs to be left out for the use of guests than a spanner, screwdriver or hammer! Anyone whose toilet does often get blocked needs to make up their minds whether they would rather have their guests try to unblock it themselves or come to them to report the problem. Having decided, put up a sign saying that unfortunately blocks do randomly happen (makes them feel it wasn't necessarily their fault, so they don't have to be embarrassed) and what they should do if one does, e.g. 'Try flushing twice more. If that doesn't clear it, let us know / use the plunger below this notice'.