I think you have some inflated notion of what a "cluster" is. A distributed cache, where any server has access to the information stored by another, *is* a cluster. To enable distributed web sessions, all you need to do is:
1. ensure your web application define <distributable/> in their web.xml
2. start your servers using the standalone-ha.xml instead of standalone.xml (or standalone-full-ha.xml instead of standalone-full.xml)
In most cases, the default configuration works just fine.