A general observation: when setting up a dual radio device as wireless repeater, it is important to have separation between both radios:
- use (directional) antennae oriented so that main antenna beams don't overlap
- make sure both used frequency channels don't overlap at all
The main reason is that when these two radios operate independently (that's the idea of using two radios, right?), it can happen that one is transmitting while the other one is receiving. Due to extremely low distance between antennae attached to both radios the receiving antenna will receive extremely high signal level from the transmitting one ... effectively deafening it. If using well designed antenna so that their beams don't overlap, the problem is largely reduced. Making both radios operating on entirely non-overlapping frequencies helps even further.
Note that the second bullet (operating on different frequencies) doesn't help if the first one isn't there. The reason is that input filters are wide to cover entire 2.4GHz wifi band. And receiver's pre-amplifier works on whole band. At the same time it's using automatic gain control (AGC) which makes sure that receiver amplification is fine for highest received signal (if amplification is too high, output is distorted). Even if it's not the one digital part of receiver is interested in (this is the way also professional radios work). So it's essential to keep unwanted signal as low as possible which is not an easy task with another transmitter co-located.
This is the reason why Audience, which features two independent 5GHz radios, has limitation on frequency selection on both (one operates in lower part of 5GHz band, the other one in upper part of same band ... with quite large gap between both band parts). Since limitation is hard coded, it is possible to construct appropriate band-pass filters so that lack of implementation of first bullet doesn't exactly kill the radio performance.