You do realize you have to wait for the other grenade to recharge right? It's not like as soon as you fill up grenade energy you automatically have two grenades to throw out. While it's cool to have an extra grenade if you wait for both to come off cd, if you are a guardian titan, the helmet is MUCH better - when enemies get in your bubble they are blinded. For pve that means they are basically stunlocked which means your bubble truly is an immunity shield.