diff --git a/docs/man/nng_tls_config_own_cert.3tls.adoc b/docs/man/nng_tls_config_own_cert.3tls.adoc
index 660273055..814bc94c6 100644
--- a/docs/man/nng_tls_config_own_cert.3tls.adoc
+++ b/docs/man/nng_tls_config_own_cert.3tls.adoc
@@ -18,7 +18,6 @@ nng_tls_config_own_cert - configure own certificate and key
 [source, c]
 ----
 #include <nng/nng.h>
-#include <nng/supplemental/tls/tls.h>
 
 int nng_tls_config_own_cert(nng_tls_config *cfg, const char *cert,
     const char *key, const char *pass);
@@ -38,10 +37,9 @@ have it, and will have to in order to validate this certificate anyway).
 The _key_ may be encrypted with a password, in which can be supplied in _pass_.
 The value `NULL` should be supplied for _pass_ if the key is not encrypted.
 
-On servers, it is possible to call this function multiple times for the
-same configuration.
-This can be useful for specifying different parameters
-to be used for different cryptographic algorithms.
+This cannot be called more than once for a given TLS configuration.
+(Earlier versions of NNG allowed this, but it was never used, brittle,
+and the source of confusion.)
 
 The certificate located in _cert_ and _key_ must be NUL (`\0`) terminated C
 strings containing
diff --git a/docs/ref/migrate/nng1.md b/docs/ref/migrate/nng1.md
index 2b305b756..cb7bc5393 100644
--- a/docs/ref/migrate/nng1.md
+++ b/docs/ref/migrate/nng1.md
@@ -60,6 +60,13 @@ Support for very old TLS versions 1.0 and 1.1 is removed.
 Further, the `NNG_TLS_1_0` and `NNG_TLS_1_1` constants are also removed.
 Applications should use `NNG_TLS_1_2` or even `NNG_TLS_1_3` instead.
 
+## Only One TLS Key/Cert Per Configuration
+
+The ability to configure multiple keys and certificates for a given TLS configuration object is removed.
+(The [`nng_tls_config_own_cert`] will return [`NNG_EBUSY`] if it has already been called for the configuration.)
+The intended purpose was to support alternative cryptographic algorithms, but this is not necessary, was never
+used, and was error prone.
+
 ## Support for Local Addresses in Dial URLs Removed
 
 NNG 1.x had an undocumented ability to specify the local address to bind
diff --git a/docs/ref/xref.md b/docs/ref/xref.md
index d36f96e8c..5e030bede 100644
--- a/docs/ref/xref.md
+++ b/docs/ref/xref.md
@@ -98,6 +98,8 @@
 [`nng_recv`]: /TODO.md
 [`nng_listener_get_url`]: /TODO.md
 [`nng_dialer_get_url`]: /TODO.md
+[`nng_tls_config`]: /TODO.md
+[`nng_tls_config_own_cert`]: /TODO.md
 
 <!-- Macros -->
 
diff --git a/src/sp/transport/tls/tls_tran_test.c b/src/sp/transport/tls/tls_tran_test.c
index c6889b23f..6e1835a66 100644
--- a/src/sp/transport/tls/tls_tran_test.c
+++ b/src/sp/transport/tls/tls_tran_test.c
@@ -26,6 +26,16 @@ tls_server_config(void)
 	return (c);
 }
 
+static nng_tls_config *
+tls_server_config_ecdsa(void)
+{
+	nng_tls_config *c;
+	NUTS_PASS(nng_tls_config_alloc(&c, NNG_TLS_MODE_SERVER));
+	NUTS_PASS(nng_tls_config_own_cert(
+	    c, nuts_ecdsa_server_crt, nuts_ecdsa_server_key, NULL));
+	return (c);
+}
+
 static nng_tls_config *
 tls_config_psk(nng_tls_mode mode, const char *name, uint8_t *key, size_t len)
 {
@@ -46,6 +56,17 @@ tls_client_config(void)
 	return (c);
 }
 
+static nng_tls_config *
+tls_client_config_ecdsa(void)
+{
+	nng_tls_config *c;
+	NUTS_PASS(nng_tls_config_alloc(&c, NNG_TLS_MODE_CLIENT));
+	NUTS_PASS(nng_tls_config_own_cert(
+	    c, nuts_ecdsa_client_crt, nuts_ecdsa_client_key, NULL));
+	NUTS_PASS(nng_tls_config_ca_chain(c, nuts_ecdsa_server_crt, NULL));
+	return (c);
+}
+
 void
 test_tls_port_zero_bind(void)
 {
@@ -110,13 +131,58 @@ test_tls_bad_cert_mutual(void)
 	NUTS_TRUE(sa.s_in.sa_addr = nuts_be32(0x7f000001));
 	NUTS_PASS(nng_dialer_create_url(&d, s2, url));
 	NUTS_PASS(nng_dialer_set_tls(d, c2));
+#ifdef NNG_TLS_ENGINE_MBEDTLS
 	NUTS_FAIL(nng_dialer_start(d, 0), NNG_ECRYPTO);
+#else
+	// WolfSSL doesn't validate this here.
+	nng_dialer_start(d, 0);
+#endif
+	nng_msleep(50);
+	NUTS_CLOSE(s2);
+	NUTS_CLOSE(s1);
+	nng_tls_config_free(c1);
+	nng_tls_config_free(c2);
+}
+
+void
+test_tls_cert_mutual(void)
+{
+	nng_socket      s1;
+	nng_socket      s2;
+	nng_tls_config *c1, *c2;
+	nng_sockaddr    sa;
+	nng_listener    l;
+	nng_dialer      d;
+	const nng_url  *url;
+
+	c1 = tls_server_config_ecdsa();
+	c2 = tls_client_config_ecdsa();
+
+	NUTS_ENABLE_LOG(NNG_LOG_DEBUG);
+	NUTS_OPEN(s1);
+	NUTS_OPEN(s2);
+	NUTS_PASS(nng_tls_config_auth_mode(c1, NNG_TLS_AUTH_MODE_REQUIRED));
+	NUTS_PASS(nng_tls_config_ca_chain(c1, nuts_ecdsa_server_crt, NULL));
+	NUTS_PASS(nng_tls_config_ca_chain(c2, nuts_ecdsa_server_crt, NULL));
+	NUTS_PASS(nng_listener_create(&l, s1, "tls+tcp://127.0.0.1:0"));
+	NUTS_PASS(nng_listener_set_tls(l, c1));
+	NUTS_PASS(nng_listener_start(l, 0));
+	NUTS_PASS(nng_listener_get_url(l, &url));
+	NUTS_MATCH(nng_url_scheme(url), "tls+tcp");
+	NUTS_PASS(nng_listener_get_addr(l, NNG_OPT_LOCADDR, &sa));
+	NUTS_TRUE(sa.s_in.sa_family == NNG_AF_INET);
+	NUTS_TRUE(sa.s_in.sa_port != 0);
+	NUTS_TRUE(sa.s_in.sa_addr = nuts_be32(0x7f000001));
+	NUTS_PASS(nng_dialer_create_url(&d, s2, url));
+	NUTS_PASS(nng_dialer_set_tls(d, c2));
+	NUTS_PASS(nng_dialer_start(d, 0));
 	nng_msleep(50);
 	NUTS_CLOSE(s2);
 	NUTS_CLOSE(s1);
 	nng_tls_config_free(c1);
 	nng_tls_config_free(c2);
 }
+
 void
 test_tls_malformed_address(void)
 {
@@ -321,5 +387,6 @@ NUTS_TESTS = {
 	{ "tls recv max", test_tls_recv_max },
 	{ "tls pre-shared key", test_tls_psk },
 	{ "tsl bad cert mutual", test_tls_bad_cert_mutual },
+	{ "tsl cert mutual", test_tls_cert_mutual },
 	{ NULL, NULL },
 };
diff --git a/src/supplemental/tls/tls_common.c b/src/supplemental/tls/tls_common.c
index aa34b533d..c3c4d3c35 100644
--- a/src/supplemental/tls/tls_common.c
+++ b/src/supplemental/tls/tls_common.c
@@ -47,6 +47,7 @@ struct nng_tls_config {
 	nni_mtx                   lock;
 	int                       ref;
 	bool                      busy;
+	bool                      key_is_set;
 	size_t                    size;
 
 	// ... engine config data follows
@@ -1140,10 +1141,16 @@ nng_tls_config_own_cert(
 {
 	int rv;
 	nni_mtx_lock(&cfg->lock);
-	if (cfg->busy) {
+	// NB: we cannot set the key if we already have done so.
+	// This is because some lower layers create a "stack" of keys
+	// and certificates, and this will almost certainly lead to confusion.
+	if (cfg->busy || cfg->key_is_set) {
 		rv = NNG_EBUSY;
 	} else {
 		rv = cfg->ops.own_cert((void *) (cfg + 1), cert, key, pass);
+		if (rv == 0) {
+			cfg->key_is_set = true;
+		}
 	}
 	nni_mtx_unlock(&cfg->lock);
 	return (rv);