Actually you can only have one default instance, any additional ones have to be named. This is no different from a standalone server, and is due to the fact that the service names must be unique, in case you are in a situation where multiple instances are running on the same node.

If you had 2 default instances they would both need a service called MSSQLSERVER, which is obviously not possible. Likewise 2 named instances called A would need a service called MSSQL$A.

In answer to your other question, each instance needs its own storage that is added as a resource to its cluster group. It is not possible to share storage between instances.