I'm reading from the Irish Taxation Institute's Taxation Summary for 2007 page 24 part 1.3.18 ( cant find this online anywhere or I'd link it )
It says : - "Relief is available on qualifying fees, per course, per academic year e.g., if a taxpayer has paid qualifying fees in respect of three dependents attending college in an academic year, he/she is entitled to relief up to the maximum limit in respect of each of these dependents. "
This may suggest that relief is available on both but the example is for different attendees. In your case it might be best to check with Revenue. A call to your district might help clarify. It is a slightly unusual situation and unfortunately the texts don't really seem to cover it.