Send Email to multiple recipients

With the Send Email Smart service, I am having trouble figuring out how to send to multiple users.

This works:
=touser("NetworkUserID")

This sends Alert Email:
=touser("NetworkUserID") & touser("NetworkUserID")
Also tried this:
=touser("NetworkUserID") & ";" & touser("NetworkUserID")

Alert Email:
There is a problem with task “Send Email” in the process “Test”. No valid recipients resolved
Problem: An error occurred in executing an Activity Class.
Recommended Action: Examine the activity class to correct the error and then resume.

Thanks for the help!

OriginalPostID-175276

OriginalPostID-175276