The ArcGIS geodatabase model only supports 1 class-extension per feature class or table at a time.
To enable multiple class extensions support you will need to develop a custom class extension that knows:
How to instantiate all other needed class extensions;
Hook into all class extension events that are needed;
Delegate the events to the "sub-class extensions".
This model worked in some implementations that we customized for our customers. We actually, built a dynamic class-extension register to control which extension gets instantiated at run-time and delegate everything to the appropriate .NET classes.
We are planning to open-source this control, which we called Morphling (after the Magic The Gathering card).