if you have both antennas connected then it is better to have both chains enabled for receive and transmit.
Thanks for this clarification.
Better leave the default power - do not use higher output power as that might damage the wireless card.
I am testing with 23dBm dual disk antenna's that are in my garden 10 mtrs apart. I had the cards both set to 1dBm power and still getting -24, -26dBm readings. After this chain0 is disabled I have to tune the power up to 3dBm to get it back on the other end.... when it then is back the power reads -30dBm.
So the issue would rather be the cards don' t work well at minimal output instead of high outputs....
Try to use UDP Bandwidth test as the RouterBoards can't generate so much TCP by the selves.
udp is getting up to 100Mb. But I ran download tests from a laptop to a Pc connected to the both routers and also did do an ordinary big file transfer. Best speed still 20-24Mb....