Now the NL
So 14 times out of 202 possible years (101 for each league, not counting 1994 when there were no games in September). So it happens about 6.9% of the time. This will be only the third time for both leagues in the same year. The AL came close in 2007 with a .770 OPS in September while May was highest with .771.
The second highest month in the AL so far was August with .739. In the NL, it is also August with .734. So the AL is a much better bet to have its highest OPS in September, since that month is currently .020 ahead of the next highest while it is only .009 higher for the NL.