Lift evaluates the <lift:xxx/> tags from the outside in, otherwise knows as lazy evaluation. This means that the following code will only embed a single template:

<lift:TestCond.loggedout>
  <lift:embed what="/templates/_login_panel"/>
</lift:TestCond.loggedout>

<lift:TestCond.loggedin>
  <lift:embed what="/templates/_welcome_message"/>
</lift:TestCond.loggedin>

Depending on if the user is logged in.

This also means that a snippet may return XML that contains other snippet tags.