Attaching to Specific Instances of the IDE

when I do:

// Get an instance of the currently running Visual Studio IDE.
EnvDTE80.DTE2 dte2;
dte2 = (EnvDTE80.DTE2)System.Runtime.InteropServices.Marshal.
GetActiveObject("VisualStudio.DTE.10.0");

var solution = dte2.Solution; // get solution

Console.WriteLine(solution.FullName);  // prints the name of the solution where this code is written

I am able to get an instance of the current ide

I will like to get a reference to dte2 of a different visual studio instance though. This link states that it is possible to do that. As a result I have tried something like:

    Process p = new Process();
    ProcessStartInfo ps = new ProcessStartInfo(@"C:\Program Files (x86)\Microsoft Visual Studio 10.0\Common7\IDE\devenv.exe");
    p.StartInfo = ps;
    p.Start(); // start a new instance of visual studio

    var ROT = "!VisualStudio.DTE.10.0:" + p.Id;

    // Get an instance of the NEW instance of Visual Studio IDE.
    EnvDTE80.DTE2 dte2;
    dte2 = (EnvDTE80.DTE2)System.Runtime.InteropServices.Marshal.
    GetActiveObject(ROT); 

If I try that I get the exception:

Invalid class string (Exception from HRESULT: 0x800401F3 (CO_E_CLASSSTRING))

There are more links that show how to do what am I looking for but for some reason I cannot make it work. Here are some of the links:

http://msdn.microsoft.com/en-us/library/6cefss65.aspx

http://msdn.microsoft.com/en-us/library/ms228755.aspx

One thing that has worked for me consistently is

var dte = GetDTE();
var debugger = dte.Debugger;
var processes = debugger.LocalProcesses;
int processIdToAttach; //your process id of second visual studio
foreach (var p in processes)
{
    EnvDTE90.Process3 process3 = (EnvDTE90.Process3)p;
    if (process3.ProcessID == processIdToAttach)
    {
        if (!process3.IsBeingDebugged)
        {
            if (doMixedModeDebugging)
            {
                string[] arr = new string[] { "Managed", "Native" };
                process3.Attach2(arr);
            }
            else
            {
                process3.Attach();
            }
        }
        break;
    }
}