I usually pin visual studio to the task bar and then when I fire it up I get all instances appear as a stack of cards on the pin. However I have this interesting problem now where when I start it it creates a new temporary pin and all instances of the app load up on that pin. Annoying how do I stop this happening..
Have you ever used a taskbar tweaker tool like 7 Taskbar tweaker? These tools allow you to change the default behavior. The problem you desribe is one of those options.
Try using 7 Taskbar tweaker and change this option: