diff --git a/distributed/active_memory_manager.py b/distributed/active_memory_manager.py
index 098773c224..c2bbe7ccd3 100644
--- a/distributed/active_memory_manager.py
+++ b/distributed/active_memory_manager.py
@@ -52,23 +52,20 @@ def __init__(
         interval: float | None = None,
     ):
         self.scheduler = scheduler
+        self.policies = set()
 
         if policies is None:
+            # Initialize policies from config
             policies = set()
             for kwargs in dask.config.get(
                 "distributed.scheduler.active-memory-manager.policies"
             ):
                 kwargs = kwargs.copy()
                 cls = import_term(kwargs.pop("class"))
-                if not issubclass(cls, ActiveMemoryManagerPolicy):
-                    raise TypeError(
-                        f"{cls}: Expected ActiveMemoryManagerPolicy; got {type(cls)}"
-                    )
                 policies.add(cls(**kwargs))
 
         for policy in policies:
-            policy.manager = self
-        self.policies = policies
+            self.add_policy(policy)
 
         if register:
             scheduler.extensions["amm"] = self
@@ -92,16 +89,28 @@ def __init__(
 
     def start(self, comm=None) -> None:
         """Start executing every ``self.interval`` seconds until scheduler shutdown"""
+        if self.started:
+            return
         pc = PeriodicCallback(self.run_once, self.interval * 1000.0)
-        self.scheduler.periodic_callbacks["amm"] = pc
+        self.scheduler.periodic_callbacks[f"amm-{id(self)}"] = pc
         pc.start()
 
     def stop(self, comm=None) -> None:
         """Stop periodic execution"""
-        pc = self.scheduler.periodic_callbacks.pop("amm", None)
+        pc = self.scheduler.periodic_callbacks.pop(f"amm-{id(self)}", None)
         if pc:
             pc.stop()
 
+    @property
+    def started(self) -> bool:
+        return f"amm-{id(self)}" in self.scheduler.periodic_callbacks
+
+    def add_policy(self, policy: ActiveMemoryManagerPolicy) -> None:
+        if not isinstance(policy, ActiveMemoryManagerPolicy):
+            raise TypeError(f"Expected ActiveMemoryManagerPolicy; got {policy!r}")
+        self.policies.add(policy)
+        policy.manager = self
+
     def run_once(self, comm=None) -> None:
         """Run all policies once and asynchronously (fire and forget) enact their
         recommendations to replicate/drop keys
diff --git a/distributed/tests/test_active_memory_manager.py b/distributed/tests/test_active_memory_manager.py
index afe6c11494..c7c747b850 100644
--- a/distributed/tests/test_active_memory_manager.py
+++ b/distributed/tests/test_active_memory_manager.py
@@ -101,6 +101,65 @@ async def test_auto_start(c, s, a, b):
     assert len(s.tasks["x"].who_has) == 1
 
 
+@gen_cluster(client=True, config=demo_config("drop", key="x"))
+async def test_add_policy(c, s, a, b):
+    p2 = DemoPolicy(action="drop", key="y", n=10, candidates=None)
+    p3 = DemoPolicy(action="drop", key="z", n=10, candidates=None)
+
+    # policies parameter can be:
+    # - None: get from config
+    # - explicit set, which can be empty
+    m1 = s.extensions["amm"]
+    m2 = ActiveMemoryManagerExtension(s, {p2}, register=False, start=False)
+    m3 = ActiveMemoryManagerExtension(s, set(), register=False, start=False)
+
+    assert len(m1.policies) == 1
+    assert len(m2.policies) == 1
+    assert len(m3.policies) == 0
+    m3.add_policy(p3)
+    assert len(m3.policies) == 1
+
+    futures = await c.scatter({"x": 1, "y": 2, "z": 3}, broadcast=True)
+    m1.run_once()
+    while len(s.tasks["x"].who_has) == 2:
+        await asyncio.sleep(0.01)
+
+    m2.run_once()
+    while len(s.tasks["y"].who_has) == 2:
+        await asyncio.sleep(0.01)
+
+    m3.run_once()
+    while len(s.tasks["z"].who_has) == 2:
+        await asyncio.sleep(0.01)
+
+
+@gen_cluster(client=True, config=demo_config("drop", key="x", start=False))
+async def test_multi_start(c, s, a, b):
+    """Multiple AMMs can be started in parallel"""
+    p2 = DemoPolicy(action="drop", key="y", n=10, candidates=None)
+    p3 = DemoPolicy(action="drop", key="z", n=10, candidates=None)
+
+    # policies parameter can be:
+    # - None: get from config
+    # - explicit set, which can be empty
+    m1 = s.extensions["amm"]
+    m2 = ActiveMemoryManagerExtension(s, {p2}, register=False, start=True, interval=0.1)
+    m3 = ActiveMemoryManagerExtension(s, {p3}, register=False, start=True, interval=0.1)
+
+    assert not m1.started
+    assert m2.started
+    assert m3.started
+
+    futures = await c.scatter({"x": 1, "y": 2, "z": 3}, broadcast=True)
+
+    # The AMMs should run within 0.1s of the broadcast.
+    # Add generous extra padding to prevent flakiness.
+    await asyncio.sleep(0.5)
+    assert len(s.tasks["x"].who_has) == 2
+    assert len(s.tasks["y"].who_has) == 1
+    assert len(s.tasks["z"].who_has) == 1
+
+
 @gen_cluster(client=True, config=NO_AMM_START)
 async def test_not_registered(c, s, a, b):
     futures = await c.scatter({"x": 1}, broadcast=True)