Those 2 omni antennas should be in different polarization (one horizontal, second vertical),
otherwise it won't be any better than just one antenna.
It's much better to use 3 or 4 sector AP devices like those from SXT series (http://routerboard.com/RBSXTG-5HPnD-SAr2
If this is too expensive then it's better to use Omnitik (http://routerboard.com/RBOmniTIKU-5HnD
) than trying to invent one.
p.s. for both mmcx connectors to work you should enable both chains on the card (in winbox wireless wlanx
see tab named "HT" with 4 check-fields, by default only ht-tx-chain0 and ht-rx-chain0 are enabled,
you'll need to enable and other 2 check-fields for chain1).