diff --git a/src/subscriber/subscriber.ml b/src/subscriber/subscriber.ml
index ef2dd77..8ecf0a8 100644
--- a/src/subscriber/subscriber.ml
+++ b/src/subscriber/subscriber.ml
@@ -8,42 +8,89 @@ type t =
 
 let dummy : t = Sub { st = (); callbacks = Callbacks.dummy () }
 
-(* TODO:
-   let multiplex (l : t list) : t =
-     match l with
-     | [] -> dummy
-     | [ s ] -> s
-     | _ ->
-       let module M = struct
-         type st = t list
-
-         let on_init l ~time_ns =
-           List.iter
-             (fun (Sub { st; callbacks = (module CB) }) -> CB.on_init st ~time_ns)
-             l
-
-         let on_shutdown _ ~time_ns:_ = ()
-         let on_tick _ = ()
-         let on_name_thread _ ~time_ns:_ ~tid:_ ~name:_ = ()
-         let on_name_process _ ~time_ns:_ ~tid:_ ~name:_ = ()
-         let on_message _ ~time_ns:_ ~tid:_ ~span:_ ~data:_ _msg = ()
-         let on_counter _ ~time_ns:_ ~tid:_ ~data:_ ~name:_ _v = ()
-
-         let on_enter_span _ ~__FUNCTION__:_ ~__FILE__:_ ~__LINE__:_ ~time_ns:_
-             ~tid:_ ~data:_ ~name:_ _sp =
-           ()
-
-         let on_exit_span _ ~time_ns:_ ~tid:_ _ = ()
-         let on_add_data _ ~data:_ _sp = ()
-
-         let on_enter_manual_span _ ~__FUNCTION__:_ ~__FILE__:_ ~__LINE__:_
-             ~time_ns:_ ~tid:_ ~parent:_ ~data:_ ~name:_ ~flavor:_ ~trace_id:_ _sp
-             =
-           ()
-
-         let on_exit_manual_span _ ~time_ns:_ ~tid:_ ~name:_ ~data:_ ~flavor:_
-             ~trace_id:_ _ =
-           ()
-       end in
-       Sub { st = l; callbacks = (module M) }
-*)
+open struct
+  module Tee_cb : Callbacks.S with type st = t * t = struct
+    type nonrec st = t * t
+
+    let on_init
+        ( Sub { st = s1; callbacks = (module CB1) },
+          Sub { st = s2; callbacks = (module CB2) } ) ~time_ns =
+      CB1.on_init s1 ~time_ns;
+      CB2.on_init s2 ~time_ns
+
+    let on_shutdown
+        ( Sub { st = s1; callbacks = (module CB1) },
+          Sub { st = s2; callbacks = (module CB2) } ) ~time_ns =
+      CB1.on_shutdown s1 ~time_ns;
+      CB2.on_shutdown s2 ~time_ns
+
+    let on_name_thread
+        ( Sub { st = s1; callbacks = (module CB1) },
+          Sub { st = s2; callbacks = (module CB2) } ) ~time_ns ~tid ~name =
+      CB1.on_name_thread s1 ~time_ns ~tid ~name;
+      CB2.on_name_thread s2 ~time_ns ~tid ~name
+
+    let on_name_process
+        ( Sub { st = s1; callbacks = (module CB1) },
+          Sub { st = s2; callbacks = (module CB2) } ) ~time_ns ~tid ~name =
+      CB1.on_name_process s1 ~time_ns ~tid ~name;
+      CB2.on_name_process s2 ~time_ns ~tid ~name
+
+    let on_enter_span
+        ( Sub { st = s1; callbacks = (module CB1) },
+          Sub { st = s2; callbacks = (module CB2) } ) ~__FUNCTION__ ~__FILE__
+        ~__LINE__ ~time_ns ~tid ~data ~name span =
+      CB1.on_enter_span s1 ~__FUNCTION__ ~__FILE__ ~__LINE__ ~time_ns ~tid ~data
+        ~name span;
+      CB2.on_enter_span s2 ~__FUNCTION__ ~__FILE__ ~__LINE__ ~time_ns ~tid ~data
+        ~name span
+
+    let on_exit_span
+        ( Sub { st = s1; callbacks = (module CB1) },
+          Sub { st = s2; callbacks = (module CB2) } ) ~time_ns ~tid span =
+      CB1.on_exit_span s1 ~time_ns ~tid span;
+      CB2.on_exit_span s2 ~time_ns ~tid span
+
+    let on_add_data
+        ( Sub { st = s1; callbacks = (module CB1) },
+          Sub { st = s2; callbacks = (module CB2) } ) ~data span =
+      CB1.on_add_data s1 ~data span;
+      CB2.on_add_data s2 ~data span
+
+    let on_message
+        ( Sub { st = s1; callbacks = (module CB1) },
+          Sub { st = s2; callbacks = (module CB2) } ) ~time_ns ~tid ~span ~data
+        msg =
+      CB1.on_message s1 ~time_ns ~tid ~span ~data msg;
+      CB2.on_message s2 ~time_ns ~tid ~span ~data msg
+
+    let on_counter
+        ( Sub { st = s1; callbacks = (module CB1) },
+          Sub { st = s2; callbacks = (module CB2) } ) ~time_ns ~tid ~data ~name
+        n =
+      CB1.on_counter s1 ~time_ns ~tid ~data ~name n;
+      CB2.on_counter s2 ~time_ns ~tid ~data ~name n
+
+    let on_enter_manual_span
+        ( Sub { st = s1; callbacks = (module CB1) },
+          Sub { st = s2; callbacks = (module CB2) } ) ~__FUNCTION__ ~__FILE__
+        ~__LINE__ ~time_ns ~tid ~parent ~data ~name ~flavor ~trace_id span =
+      CB1.on_enter_manual_span s1 ~__FUNCTION__ ~__FILE__ ~__LINE__ ~time_ns
+        ~tid ~parent ~data ~name ~flavor ~trace_id span;
+      CB2.on_enter_manual_span s2 ~__FUNCTION__ ~__FILE__ ~__LINE__ ~time_ns
+        ~tid ~parent ~data ~name ~flavor ~trace_id span
+
+    let on_exit_manual_span
+        ( Sub { st = s1; callbacks = (module CB1) },
+          Sub { st = s2; callbacks = (module CB2) } ) ~time_ns ~tid ~name ~data
+        ~flavor ~trace_id span =
+      CB1.on_exit_manual_span s1 ~time_ns ~tid ~name ~data ~flavor ~trace_id
+        span;
+      CB2.on_exit_manual_span s2 ~time_ns ~tid ~name ~data ~flavor ~trace_id
+        span
+  end
+end
+
+let tee (s1 : t) (s2 : t) : t =
+  let st = s1, s2 in
+  Sub { st; callbacks = (module Tee_cb) }