I'm taking the following text from the NCAA statistician manual (Basic Interpretations section):

ASSISTED TACKLE: If two defensive players contribute equally to the stopping of a ball carrier, credit both players with an assisted tackle, but give neither a solo tackle.
NOTE: In a situation where there is not a solo tackle, there should not be more than two assists credited except in rare instances. In other words, on any given play one of the following may be credited: (1) one solo tackle, or (2) two assisted tackles. Only in the rarest of cases would more than two assisted tackles be credited.


Since the manual states that it can happen on rare cases (though they don't really define what those cases are) we have the option. It credits each player with an assisted tackle (.5).