Cactus has modified visibily leaves called spine. These spines help and protect cactus from human and animal harm and also reduce the water loss from the cactus stem. When there is less rain the roots spread out and store water in the stem.
I think they will grow petter alone if they are not planted in the same pot. The one plant will absorb more nutrients from the soil it is planted in rather than if it was planted with another.
