How to combine signals from multiple antennas

I am trying to follow this guide, which explains how to estimate the direction of arrival for one or more signals. I assume it is doing this from comparing the phase difference between the signals received from each of the antennas.

If this is the case however, I can't work out how the signals from each of the antennas are included. It talks of a 'multichannel signal', but am unclear if that means I need to somehow combine the signal from the different antennas beforehand, and if so, how does it recognise this?

My current setup is using two separate receiver devices, each with one antenna each (though I'm looking at getting a signle receiver device with multiple antennas built-in).

Any guidance would be much appreciated!

Thanks so much :)