What you are talking about is called vertical clustering. That is...federating multiple nodes on the same box into the same cell. There are actually two ways you can do this. Keep in mind, if you're going to run a vertical cluster, your machine better be pretty strong because even one WAS instance is very processor and memory hungry. Also keep in mind that vertical clustering is a SPOF (single point of failure) and will only possibly aid in work load management (that is if the machine is quick enough to run all the nodes at once, a multiprocessor machine really is a must).
The first, and my personal recommendation if this is a production environment is to create a "coexistence environment." Basically, install WAS base on the machine for each additional node you want. Then federate these nodes into the cell created by NDM. Here is a good page to learn more about coexistence:
http://publib.boulder.ibm.com/infocenter/wasinfo/v5r1//index.jsp?topic=/com.ibm.websphere.base.doc/info/aes/ae/rins_coexist.html The second method is to create multiple configuration instances. This method is good for development scenarios or a box with cramped hard drive space. In this scenarion you install WAS base once and then run the wsinstance command to create multiple configuration instances. These instances can than be federated into the cluster. Now for some reason I thought I remember reading somewhere that you can't federate a configuration instance into a cell, but I know for a fact that is untrue because I do it all the time at work when developing. However, multiple configuration instances can be rather confusing at first, so keep that in mind. Here is a good page about multiple configuration instances:
http://publib.boulder.ibm.com/infocenter/wasinfo/v5r1//index.jsp?topic=/com.ibm.websphere.base.doc/info/aes/ae/tins_instances.html. Also, another things to always keep in mind are port numbers! You will run into a lot of problems if your port numbers conflict. For example, if you have global security on and one of your listener ports is already binded by another application you will recieve an error about your keystore being no good, but in reality it has nothing to do with your keystore. It would be wise to come up with a port numbering scheme and store it in a text file somehwere.
Most of all, goodluck, because sometimes when working with WAS you just need to cross your fingers and hope to god it works.