multiservemux.go

// routes1.go
func Routes1(mux *http.ServeMux) {
	mux.Handle("/route1/a", handler1A)
	mux.Handle("/route1/b", handler1B)
	mux.Handle("/route1/c", handler1C)
}

// routes2.go
func Routes2(mux *http.ServeMux) {
	mux.Handle("/route2/x", handler2X)
	mux.Handle("/route2/y", handler2Y)
	mux.Handle("/route2/z", handler2Z)
}

// combine_routes.go
func CombineRoutes(routes func(mux *http.ServeMux)...) *http.ServeMux {
	mux := http.NewServeMux()
	for r := range routes	{
		r(mux)
	}
	return mux
}

// server.go
mux := CombineRoutes(Routes1, Routes2)