That was the same equipment I was going to advise until I noticed the 300+300...
That are rather poor numbers. That equipment is capable of much more, (that is, if spectrum allows)
Something I would try is mounting the mANTs in slant mode, see
http://i.mt.lv/routerboard/files/DPA-SL ... 152145.pdf
mANT30 is a
very directive antenna, are you sure they are correctly aligned, and not main lobe - secondary lobe or similar? Can you post a Wireless > registration signal tab screenshots?
Using 2 radios on each end on a crowded spectrum would be of no use, a different story would be if spectrum is clean, or if you can allocate two at least 40MHz contiguous "spaces" in seperate areas of the spectrum.
Using high gain antenna is actually an advantage, as the lower you set Tx power, the more SNR you will get as signal is provided by the antenna gain.
If spectrum is so saturated and this were a commercial backhaul, your best bet for 600Mbps aggregated would be going to 10GHz or 24GHz, the PTP being 2km makes that even more suitable, but costs would be rather high for personal use...