To specify an alternate account (from a main account page), add the following to your user page:
{{altacc topicon
| username of alternate account
| userpage of alternate account
}}
To specify a main account (from an alternate account page), add |alt=yes
, i.e.:
{{altacc topicon
| alt=yes
| username of main account
| userpage of main account
}}
Specifying |nocat=yes
will prevent adding categories to the userpage.