Really depends on the type of gear and it is mounted - in most cases, a cantilever shelf works great if your equipment is front mount, and a center weighted shelf is generally better if your gear is center mounted. In a lot of cases, we found that we really didn't even need shelves as long as the equipment is mounted properly with rail kits etc.
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For a 2 post system I would use center mounted shelves and center mount as much as you can to keep the center of gravity down the posts. If you can only get shelves that extend out from the posts. I would mount one shelf one side the next shelf down the other side and so on to keep the center of gravity down the posts.