You need only one SkeletonJson. It loads SkeletonData instances, which can be used to create any number of Skeletons. If you have 2 skeletons you have 1 SkeletonJson, 2 SkeletonDatas, and 2 or more Skeletons.
You can resize the skeletons by setting the scale on the root bone or setting the scale on the SkeletonJson.
The enterFrame stuff is how Corona runs code every frame. I'm not sure if there is another way. You might ask on the Corona forums.
There are currently no docs, sorry. You can look at the examples and read the code. Oh, there is this diagram that shows how things fit together:
http://esotericsoftware.com/spine/files ... iagram.png