Combining two omni antennas around a single pole is not a good idea.
This configuration will produce a heavily interferred radiation pattern.
It is best to position a single antenna at either 1/4 or 1/2 lambda from the centre pole.
- At a 1/4 lambda the antenna will be most omni but with a directivity opposite of the pole like you draw on the left picture.
- at 1/2 lambda the radiation pattern will be slightly cigar-shaped radiation pattern under a 90 degree angle to the arm on which the antenna is installed.
Depending on the service area of your gateway I would choose one of these two options.
The radiation pattern of a omni antenna 1/4 lambda before the mast:
The radiation pattern of a omni antenna 1/2 lambda before the mast:
Both antennas are on an arm aiming north on these pictures.
The antenna in the following image is used for a gateway in the North of our city and is aming to the city-centre. The red arrow is the gateway antenna