fdef-:fn.clj

(require '[clojure.spec.alpha :as s]
         '[clojure.spec.test.alpha :as s-test])

(s/fdef inner
  :args (s/cat :i int?)
  :ret int?)

(defn inner [i] i)

(s/def ::result-1 int?)
(s/def ::result-2 int?)

(s/fdef outer
  :args (s/cat :i int?)
  :ret (s/keys :req [::result-1
                     ::result-2])
  ;; This fn will fail because the values generated by the calls to
  ;; inner will differ.
  :fn #(= (-> % :ret ::result-1)
          (-> % :ret ::result-2)))

(defn outer [i]
  (let [result-1 (inner i)
        result-2 (inner i)]
    {::result-1 result-1
     ::result-2 result-2}))

(s-test/instrument `inner {:stub #{`inner}})

(s-test/check `outer)

Update: @colinkahn on Clojurians slack recommended using :replace instead of :stub.