The follow is documented and supported:-
Connect the failover link in one of the following two ways:
•Using a switch, with no other device on the same network segment (broadcast domain or VLAN) as the failover interfaces of the ASA.
•Using a crossover Ethernet cable to connect the appliances directly, without the need for an external switch.
I don't think many people have configured failover as you intend to do, it is not one of the above recommended methods, so it won't be supported. I suggest you lab this and run a packet capture to determine this for yourself or implement one of the methods above.