Well you piqued my interest and had to try it myself. I programmed my NX200 to your 2tone settings and set the radio off with my service monitor. For the tone sets I used the standard 1second/3 second timing and started with a 3 second gap time between sets. No problem decoding either. Swapped tone sets for encoding and still both alerted. I reduced the gap time between tone sets a few times until I got to zero. All four tones went out mashed together and the radio could still decode both, even when the set were swapped again. Played with the auto reset timer, switched the decode sequence to A-B/C-A, swapped tones around still all good.
The only time I could get the second set of tones to fail was with the gap time at zero and I changed the alert tone timing so both were very long slow baaleeeps of several seconds. Encoding A-B/C-D to the radio worked but C-D/A-B did not. Increasing the gap time to 50ms then both worked fine.
So to me it looks like the NX200 can decode a 2tone just about any which way it comes out. Starting to think there may be problems on the way it comes across on the radio. Wide/narrow issue? Tone volume (deviation) too high or too low? I know you're probably entering tones off of a list but are these the true tones or maybe a little off (tones vary from encoder to encoder)?
I guess, again, you could put different A-B sets in two different channels to see if they will decode all by themselves. Sorry but not finding much here at my end.