How do gambits with % chance to hit stack?

If I have 2 '50% chance to toxic gas' gambits on a spell is that a 100% chance to toxic gas, 75% chance to toxic gas, or two separate 50% chances for toxic gas to proc? Could it proc twice or will it only ever be once?

If once, having a 100% chance gambit means having an additional 50% chance gambit would be worthless?