In one armed concentrator mode you only have one uplink, so you can never experience a dual active issue if a VPC peer switch would fail because both up and downlinks would be gone on a single appliance.   However VPC is in itself cannot be used in this case because the MX does not support port-channels. Individual links to both MX'es is okay though. ... View more

The thing to remember about a FEX is that it works just like a module in a chassis switch.  When connecting a FEX, the configuration is done on the parent switch.  In your case, you have a FEX connected to 2x 9ks so both 9ks are acting as the parent.  Both 9ks have to have the same identical configuration for the FEX.   The below link discusses configuration sync and Active/Active FEX topologies.  It is specifically for the N5k, but the theory and design should be similar for N9k.   https://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus5000/sw/operations/n5k_config_sync_ops.html   Config-sync is how you can synchronize the FEX configuration between the 2 9ks as long as they are running as a vPC pair. ... View more