Vista - unable to start more copies of a process under alternate credentials

Hello I use vista home premium the service "alternate logon" is started
automatically I would like to run more copies of the same program but with
different credentials, that was easy with win2000 I just checked the "run
under alternate credentials" box in the program icon options and I was
prompted each time for a user name and password and so I was able to run 2
copies of the same program at the same time (very usefull if you use for
instance 2 different IDs with a messaging software), now I can't find the
"run under alternate credentials" box, I tried to run the first copy of the
program normally and then use runas /user:user2 program.exe to open a second
copy, it prompts me for the password of user2 but then it only opens again
the program that I've just run normally rather than starting a new copy of
it. Of course I can run different copies of the same program just switching
between users but that is not usefull as then I continuosly have to switch
between users to use the different copies of the same program while I want
to be able to use them from the same desktop without switching user. Any
help appreciated. Thanks