This video was very helpful for me when attempting to make use of this feature:
I would imagine it is possible to transmit sample both ways by configuring each device as both sender and receiver as detailed within.
I believe for your example the following settings would be appropriate:
On unit A: Detect changes on DIO0
IC = 0x01
D0 = 3
D1 = 4 or 5
On unit B: Detect changes on DIO1
IC = 0x02
D0 = 4 or 5
D1 = 3
Ensure that both units allow the other to cause changes on the IO pins, that is the IA setting on each contains either the MY of the other unit, or 0xFFFF for any unit.
The video also details sending regular samples rather than just samples on change, which may be what you're looking for.
I hope this helps!