How do you null interference with beamforming?
Nulling is a RF technique for dealing with sources of interference.  A boring example might be the noise coming off of a blender motor.   When you null an interference source out, you identify the direction it comes from and then adjust the relative phases between the antennas such that signals from that particular direction interfere destructively, canceling out the noise.

The degrees of freedom are key because you only have so many knobs to twiddle.  If you have a 2x2 system, you can adjust four phases, but what will matter for the purposes of nulling is the relationship between the phases: you have 3 relative phases that you can adjust.  If you have one source of interference, its direction is well defined by an azimuth and elevation, so you have enough degrees of freedom to put a null at exactly that point.

In the case of two interference sources, you have to put 2 nulls into your directional antenna response.  Each one has an azimuth and an elevation, so your problem has 4 degrees of freedom.  Unfortunately, you only have 3 degrees of freedom in your antenna, so it is impossible for just the receiver to cancel out both signals.  With a more advanced antenna pattern, you may be able to cancel out more.

I'm not familiar with cancellation where both transmitter and receiver cooperate to nullify multiple interference sources, so you'll need to look at the literature to better understand how that works.  However, hopefully that is enough to get you started!