Hopefully something like this would do it:
esxcli network vswitch standard portgroup policy failover set --active-uplinks=vmnic0 --portgroup-name=iSCSI1
esxcli network vswitch standard portgroup policy failover set --active-uplinks=vmnic1 --portgroup-name=iSCSI2
vmnics that are not specified in the --active-uplinks syntax will be moved to unused...
/Rubeck