I think that the two antenna outputs got a reason, and is not about who antenna transmit and who receive.
At radio systems you got a coverage problem when the frecuencies goes very high, and this situation do not correct using more power at the transmiter or at the receiver. This is an signal angle problem and is so complex that the signal loose all the power is like it pass for virtual 0 degree and loose itself.
The only way to correct this situation is use another antenna equal to the first but in a different polarization angle, at cellular systems they use a second receiver w/a second receiver antenna to correct this sum both receivers signals.
At the PDF the diversity antenna algorithm is included on the chip firmware, but the antenna switch is external via PIN/GAS diodes, this antenna switch is controlled via the GPIO PINS, that means that this SW is EXTERNAL and is not included on the RFM22/23 PCB.
Build a antenna SW at this frecuencies will be very hard!
Best Regards
Frank