Today whilst I was trying to park my 2004 VW Passat B5.5, I noticed that something to do with my wing mirrors wasn't working quite right. When the toggle was on the left position, the left wing mirror would move as expected. When the toggle was on the right position (see picture), the right mirror would also move as expected. However, whenever the right mirror moves, the left one also moves in the same direction!
How do I get this control on my car to function properly?
Definitely a knackered switch. The knob probably switches the common earth for the motors in each mirror so it looks like it's earthing both mirrors when it's in the R position. Like @Paulster2 said, you need a new switch. Try and find a service manual (there's usually PDFs floating around on the Internet) so you've got instructions for getting the door card out (if required) and, more importantly, putting everything back together again.
I owned a similar generation golf and I believe this is the intended function, I think it even mentions it in the manual somewhere. Not sure why they did it but you just have to remember to adjust the side that moves both first.
