Two diodes in parallel for over-voltage protection
It won't work -- if you put 2 diodes in parallel, I can't easily say how they will share current in the forward (conducting) direction, but in blocking, the LOWEST breakdown one will breakdown. The 2nd is useless. 

You might put 2 in series, but this has its own problems (they don't split the applied V equally). Best to get a single component rated for what you want to do.