seems to work

ab.rkt

#lang racket

(module a-sig racket
  (provide a^)

  (define-signature a^
    (a)))

(module b-sig racket
  (provide b^)

  (define-signature b^
    (b)))

(module a-impl racket
  (require (submod ".." a-sig)
           (submod ".." b-sig))
  (provide a@)
  
  (define-unit a@
    (import b^)
    (export a^)
    
    (define (a x)
      (if x 'a (b x)))))

(module b-impl racket
  (require (submod ".." a-sig)
           (submod ".." b-sig))
  (provide b@)
  
  (define-unit b@
    (import a^)
    (export b^)
    
    (define (b x)
      (if x (a x) 'b))))

(require 'a-sig
         'a-impl
         'b-sig
         'b-impl)

(define-compound-unit/infer ab@
  (import)
  (export a^
          b^)
  (link a@
        b@))

(define-values/invoke-unit/infer ab@)

(a 1)
(a #f)
(b 1)
(b #f)