One thing: do not mix number of channel and receiver: the current gateway chip (based SX1301) have 8 channels and 8 receivers so you can only ever only receive 8 packet at the same time, but the next chip (SX1302) has 16 modems for 8 channels, so with this kind of gateway you can actually receive up to 16 frame at the same time supposing they are using a unique combination of channel and SF.
And about the use of 2 gateway monitoring the same frequencies: you could see clear benefits if you limit the spreading listen by each gateway: for example one for SF 5 to 8 and another for SF 9 to 12.