Wolf, when Beis Din set the chodesh on a month by month basis rather than the current calendrical cycle, they still knew early enough when it would be a leap year that celebrating Purim during two separate months didn’t happen, as the leap year was declared in the beginning of Adar.
If I’m not mistaken. (It’s happened.)
Also remember that Purim only began during the period between the two Beis Hamikdash’s. I mention this since the procedure for declaring leap years changed in that interval.