How do I pass a user's first name and last name from a user profile page to the title in _layout?
Since you're just expecting one, you should only look for one. Then you don't have to use a for loop. Plus, fullName is already an attribute of user models. So you can do this for the title:

{% set thePartner = craft.users.username( craft.request.getSegment(2) ).first() %}
&lt;title&gt;{{ thePartner.fullName }} | {{ siteUrl }} &lt;/title&gt;


If you're still having trouble, it may be with what segments you are pulling from the URL.