Two antennas, one board
It depends on where the receiver is located. 

The basic thing here is called constructive or destructive interference. 

If you throw two stones simultaneously into water, you will see an interference pattern of waves. 

Some locations will have double wave amplitude (constructive) while others will have no wave amplitude (destructive). 

For the antenna case, there will be locations that have a gain of +3dB (double power).