Parallel Stacks is a very handy feature in Visual Studio for debugging multi-threaded applications. I also use this sometimes when investigating dump files for managed applications. It provides a nice graphical view of what various threads are doing in your application. As an example, this makes it easy to find the unique stacks in your application.
Of course you can zoom into any callstacks to see more details.
However there are times when I needed textual form of all the callstacks. Windbg provides commands like ~*k or ~* e !clrstack to determine that. If you have to do the same within Visual Studio, this is where Command Window can come in handy. You can reach it using View -> Other Windows -> Command Windows menu item as shown below.
Once in command window, you can use Debug.ListCallStack command with AllThreads switch and it will spit out the available callstacks.
Until next, happy debugging!!