diff --git a/tests/wl-smoke-test.sh b/tests/wl-smoke-test.sh
index f32fa52..a5c62b3 100755
--- a/tests/wl-smoke-test.sh
+++ b/tests/wl-smoke-test.sh
@@ -34,6 +34,8 @@
 #      wManageWindow; wWindowFocus/Unfocus guarded on frame!=NULL
 #  16. (Phase 50) client_get_pid returns real PID via wl_client_get_credentials;
 #      logged as pid= in wManageWindow succeeded line
+#  17. (Phase 51) two simultaneous clients both managed and unmanaged cleanly;
+#      compositor survives multi-client scenario
 #
 # This test grows as wl_backend vtable slots are implemented.
 #
@@ -196,6 +198,26 @@ if command -v weston-simple-shm >/dev/null 2>&1; then
     [ -n "$PID" ] && [ "$PID" -gt 0 ] 2>/dev/null \
         || die "compositor did not log a valid pid= for managed toplevel"
     echo "  pid check: client PID=$PID retrieved via wl_client_get_credentials"
+
+    # -- 15. Phase 51: two simultaneous clients both managed --------------
+    #
+    # Launch two weston-simple-shm processes in parallel (2s and 3s).
+    # Both should be managed and focused; both should unmap cleanly.
+    (WAYLAND_DISPLAY="$WL_SOCK" timeout 2s weston-simple-shm 2>/dev/null) &
+    C1=$!
+    (WAYLAND_DISPLAY="$WL_SOCK" timeout 3s weston-simple-shm 2>/dev/null) &
+    C2=$!
+    wait "$C1" || true
+    wait "$C2" || true
+    sleep 0.5  # let compositor log unmap events
+
+    kill -0 "$WM_PID" 2>/dev/null \
+        || die "compositor crashed during two-client test"
+
+    MANAGED_COUNT=$(grep -c 'wManageWindow succeeded' /tmp/wl-smoke-stderr || true)
+    [ "$MANAGED_COUNT" -ge 3 ] \
+        || die "expected >=3 wManageWindow succeeded entries (1+2 clients), got $MANAGED_COUNT"
+    echo "  multi-client check: $MANAGED_COUNT toplevels managed across both sessions"
 else
     echo "  client check: skipped (weston-simple-shm not installed)"
 fi