VMWare has been designed to use 1:1 port bindings to enable Round Robin or even other MPIO if using other various SAN systems. I would think if you put the other one in standby and you had a failure you'd have some really bad thrashing going on if not a disconnect. Obviously if you have more than two vmnics on your host than add more iSCSI ports I think up to 8 was the max at least for v4.1 as I'm still new with v5.1 and currently don't need more than two per host myself.
Regards, CSSMP