FPv2: don't multiplex second panda (#27663)

* fix this way

* or can fix this way

* simplify

* fix

selfdrive/boardd/boardd.cc

@@ -127,8 +127,8 @@ bool safety_setter_thread(std::vector<Panda *> pandas) {
 
     bool obd_multiplexing_requested = p.getBool("ObdMultiplexingEnabled");
     if (obd_multiplexing_requested != obd_multiplexing_enabled) {
-      const uint16_t safety_param = obd_multiplexing_requested ? 0U : 1U;
       for (int i = 0; i < pandas.size(); i++) {
+        const uint16_t safety_param = (i > 0 || !obd_multiplexing_requested) ? 1U : 0U;
         pandas[i]->set_safety_model(cereal::CarParams::SafetyModel::ELM327, safety_param);
       }
       obd_multiplexing_enabled = obd_multiplexing_requested;