KMA20
The 40dB attenuation of inputs on a KMA20 only applies to audio loads going to the speaker isolation amplifier. Phone audio is routed just through the selector switch directly to the headphone output so multiple loads can end up being parallelled which isn't good. This is why it's necessary when using mixed-manufacture radios and an SPA400 intercom with a KMA20, to put external loading resistors into the output of some radios (220 Ohm in series will do the job) to prevent them loading the intercom with a resultant very low intercom level.
Yiour best solution is probably to ditch the KMA20 and SPA400 and fit a Garmin GMA340 or similar with a decent phone isolation amplifier and also get the benefit of a much better quality intercom. It still may not solve the issue of compatibility between units where one box breaks into spontaneous oscillation due to picking up RF from the other box on transmit - proximity of the boxes to each other can be enough to set thos off, in which case change the TKM to something with a Tx mute facility which will work with the GNS430 perhaps an SL30 or a late model second-hand KX155.