I thought the same thing then ended up with 3Well, my goal was to keep one at least partially filled with RO water. Would be hard to lift that up to stack the 2nd underneath [emoji22]
1. Always have RO on hand for top offs.
2. Always have salt water on hand for water changes
3. Need to put the change out water somewhere so that is the 3rd I do not have a drain near my tanks if I did I might not need the 3rd one.