From what I've seen they they will put out "buds" off of existing tissue at the base. If you attach the frag to a larger rock I would think that the will eventually spread to it. However if it's in the sand I'm not sure.
If they are in the sand, they will grow skeleton to support the new buds. I glued my single head frag to a rock and it has lots of real thin tiny heads sreading out on the rock. Not the best picture, but it will give you and idea.
Feed the acans to speed up growth. Decent water, flow and lights. They aren't very demanding. I've got some in my 29g under 130w of PC's and I feed them mysis evey other night and they're growing like crazy.