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.