You can have multiple modules in a single file, and you can re-open the already defined module:
(define-module (a)
#:export (a-x))
(define a-x 'a)
(define-module (b)
#:export (b-x))
(define b-x 'b)
(define-module (a)
#:use-module (b)
#:export (a-y))
(define a-y b-x)
When saved as a.scm, you can ,use (a), and get both a-x and a-y. And, iff you first ,use (a), you can even ,use (b). Interesting. Not sure how useful.