No. Not unless both antennas are connected to the same (I mean the exact same) antenna terminal of your AP/bridge.
At best, you'd want to look at something like "sector" antennas to get the additional gain (like four 90 degree sector antennas to cover the 360 horizon).
Unfortunately, you'd also need some "power dividers" to accurately distribute the TX power and accurately combine the RX signals ... then the dividers will eat about half the gain (in some cases).
*Most* of the time, for point-to-multipoint, the "point" end (the root) should be an OMNI.
Good Luck
Scott
