How to include multiple vb/cs files in the source

You can do this using assembly directives.

<%@ assembly src='test1.vb' %> 
<%@ assembly src='test2.vb' %> 

or

<%@ assembly src='test1.cs' %> 
<%@ assembly src='test2.cs' %> 

However, note that each source file will be compiled individually into its own assembly, so they cannot have dependencies on each other.