It's doesn't stack.
You can now use fusion grenades with the touch of flame perk which makes no sense since if you stick anyone with it they will die. I can see this being used in pve though. I'd rather just have any other grenade though.
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Well you can turn the warlock into a dot machine now. It free's touch of the flame. When your in radiance, and you have a solar weapon radiance ignites targets. So vex+radiance = burn. Plus you get to keep your 2nd grenade.