This template links to one person's accounts, here and on another Wiki-project.
There are three expected parameters, which may be named or just used in sequence:
User= The username (without the "User:" prefix) on this project.
Project= The name of the other project.
Alt= The username (without the "User:" prefix) on the other project. (This is only required if different from User=; it can be omitted if the same username is used for both projects.)
If you want to omit the second project, simply omit the second and third parameters; or use Project= (end of field, no value provided).