Complex sentence: One of them, Loyola University Maryland, has announced it will leave for the Patriot League after the 2012--13 school year, while Quinnipiac University and Monmouth University announced on December 14th, 2012, they will be joining the MAAC on July 1st, 2013.
Split into simpler sentences: One of them, Loyola University Maryland, has announced it will leave for the Patriot League after the 2012--13 school year, while Quinnipiac University and Monmouth University will became new members on that same season. On December 14th, 2012, Monmouth University and Quinnipiac University announced they will be joining the MAAC on July 1st, 2013.