http://forums.usfirst.org/showthread.php?t=8183 (emphasis mine)
Rule <G10> states that “a Trackball that has crossed its own Finish Line, while contacting Robots of both Alliances as it is crossing, will earn 2 points.” Note that there is no requirement for simultaneous contact with the Robots from the two Alliances. In this example, the two opposing Robots made contact with the Trackball while it was in the process of crossing the Finish Line. The requirements of the rule were satisfied. Therefore, the Trackball would still score 2 points.