$query = $this->modx->newQuery('modUser');
$query->innerJoin('modUserProfile','Profile');

//Select ALL fields from both tables
$query->select(array('modUser.*, Profile.*'));
---
$query->select(array('modUser.*'));
$query->select($this->modx->getSelectColumns('modUserProfile','Profile','profile_'));

//Select SPECIFIC fields from both tables
$query->select('modUser.id,Profile.email,Profile.fullname');
---
$query->select('modUser.id,Profile.email AS profile_email,Profile.fullname');
---
$query->select(array('modUser.*'));
$query->select($xpdo->getSelectColumns('className','alias','prefix_',array('email','fullname')));