He uses two fingers as well, and he's pretty clanky. Seems to me that the clank would be an advantage if you're trying to cop some Iron Maiden
It's also the setup on his bass - it's set up to accommodate his technique, which might be something that the OP is missing.
I learned a ton of Iron Maiden from ear, waaaaaaaay back in the day - I didn't have the benefit of the interwebs or Youtube or lessons or interviews, so I learned to gallop using three fingers, typically with my thumb anchored on the neck pup. What works for one might not work for everyone. I tried the two finger approach, and I'm just too ingrained with three fingers