mbe a.scm

(define-module (mbe a)
  #:use-module (oop goops)
  #:export (x y))

(define-generic x)
(define-generic y)

mbe b1.scm

(define-module (mbe b1)
  #:use-module (oop goops)
  #:use-module (mbe a)
  #:export (<a> y))

(define-class <a> ())

(define-method (y (a <a>))
  (x a))

mbe c1.scm

(define-module (mbe c1)
  #:use-module (oop goops)
  #:use-module (mbe a)
  #:use-module (mbe b1)
  #:export (<b> x))

(define-class <b> (<a>))

(define-method (x (b <b>))
  'b)

mbe test.scm

(define-module (mbe test)
  #:use-module (oop goops)
  #:use-module (mbe a)
  #:use-module (mbe b1)
  #:use-module (mbe c1)
  #:duplicates (merge-generics))

(y (make <b>))

When I put each module in their respective files, so that they are in %load-path, and type (use-modules (mbe test)) in guile, I get

While compiling expression:
ERROR: No applicable method for #<<generic> x (0)> in call (x #<<b> 23ccdd0>)

When I don't use separate modules, but just type the following in at the repl, it works

(use-modules (oop goops))

(define-class <a> ())

(define-generic x)
(define-generic y)

(define-method (y (a <a>))
  (display (x a)))

(define-class <b> (<a>))

(define-method (x (b <b>))
  'b)

(y (make <b>))