either.clj

(require '[clojure.spec.alpha :as s])
(s/def ::test (s/and (s/keys :req-un [(or ::key1 ::key2)]) 
              (fn [{:keys [key1 key2]}]
                 (not (and key1 key2)))))



;; s/or validates some of the keys
;; the (fn..) ensures that both keys CANNOT be present

user=> user=> (s/explain ::test {:key1 1 :key2 2 })
{:key1 1, :key2 2} - failed: (fn [{:keys [key1 key2]}] (not (and key1 key2))) spec: :user/test
nil

user=> (s/explain ::test {})
{} - failed: (or (contains? % :key1) (contains? % :key2)) spec: :user/test
nil
user=> 

user=> (s/explain ::test {:key1 1 })
Success!
nil

user=> (s/explain ::test {:key2 2})
Success!
nil

;; CAVEAT: nils are allowed
user=> (s/explain ::test {:key1 nil :key2 2})
Success!
nil

;; for NIL handling, the anonymous function should check if the keys are NOT present, not their value