http://mlb.mlb.com/mlb/downloads/y2014/official_baseball_rules.pdf

Page 19
"Rule 2.00 (Obstruction) Comment: If a fielder is about to receive a thrown ball and if the ball
is in flight directly toward and near enough to the fielder so he must occupy his position to receive the
ball he may be considered “in the act of fielding a ball.” It is entirely up to the judgment of the umpire
as to whether a fielder is in the act of fielding a ball. After a fielder has made an attempt to field a ball
and missed, he can no longer be in the “act of fielding” the ball. For example: an infielder dives at a
ground ball and the ball passes him and he continues to lie on the ground and delays the progress of
the runner, he very likely has obstructed the runner."

This says to me that if a fielder missed fielding a ball, he is not "in the act of fielding the ball." At most one player can actually field the ball and so you can't have two simultaneously "in the act of fielding" because one will, of neccessity, miss.