<p>I have two methods that I need to call in my WCF application from the client.</p>

<p>Authenticate(username, password)
GetUser(username)</p>

<p>Is it possible to combine these calls to avoid so many calls being sent back/forth?</p>

There are a few ways to handle this type of thing. First, you can use WCF sessions to initiate a group of commands that need to maintain some context between calls:
http://msdn.microsoft.com/en-us/library/ms733136.aspx
Secondly, you can take advantage of WCF's support for WS-Security standards to avoid the need to pass credentials as a second call:
http://msdn.microsoft.com/en-us/library/aa702565.aspx