What's the equivalent zener voltage of two parallel zeners?
The one that has the smaller voltage.  It will hog the current until it's voltage compliance brings the voltage into the range of the higher zener, which then will start to flow some current.  But even if the higher zener is flowing some current the majority will still flow through the smaller voltage unit.

Heating of the diodes also plays a role, especially if the voltage shifts with increased temperature. While the term Zener is generically applied to these diodes, the Zener breakdown (negative temperature coeff.) and avalanche break down (positive temperature coeff.) are two distinct mechanisms.