It basically doesn't matter whether you create a single vSwitch with multiple iSCSI VMkernel port groups, or one iSCSI VMkernel port group per vSwitch.
The important part is to ensure that there's a 1:1 mapping between an iSCSI port group, and a vmnic, i.e. only a single vmnic must be active per ISCSI port group, any others need to be set to unused.
Before adding additional iSCSI port groups, remember that the "Number of paths to a LUN" for iSCSI is 8.
André