Is it possible to retrieve Window Proc in process running in other user?

I am encoutering a problem on retrieving the Window Proc of a window that running in a process with another user.
For example, I have logon windows with domain\user1, and run the following command as:

runas /profile /env /user:domain\user2 C:\Windows\System32\calc.exe

And then the Calculator is shown in domain\user1 desktop, but the process is running in domain\user2 account.
When running Spy++ (either wither domain\user1 or domain\user2) to view the calculator window, the Windoe Proc field is shown 'Unavailable'
I also try to call GetWindowLong against this window, it will always returns NULL.

I am just wondering if it is possible to retrieve the Window Proc of the window running in a process of another account, is is possible?

Thanks