Is it necessary to have a waste gate and a BOV on a BNR stage 2?

 

You already have both stock. The engine would blow without a wastegate.

 

2 completely different items. A blowoff valve prevents compressor surge; a condition that exists when boosting, then you let off the gas. The throttle plate shuts, the boost pressure is still built up in the charge pipes and has nowhere to go, QUICKLY spinning the turbo down. Very bad for it.

A wastegate directs exhaust gasses around the turbo instead of through it when the boost pressure gets to a predetermined pressure. This is so you're not forcing 40psi from a turbo meant to push 15psi.

Both have very important (and different) jobs.