How are you testing? In layer 2 mode all traffic between the two same MAC addresses are on the same link. You’d need to look at throughput while several machines are transferring data.
Look at the bonding wiki or the below link for some details on how traffic is distributed. Your configuration looks fine to me.
http://forum.mikrotik.com/t/per-packet-load-balancing-supported-by-isp/47934/1