How big is the problem of two chopping frequencies beating up on each other - could that lead to low frequency misbehaviour?
I have seen it happen with older parts which did not dither their chopping clock; the symptoms typically include low frequency tones or high offset voltage drift which may be indistinguishable from low frequency noise. (1) Some of the early parts provide a way to synchronize the chopping clock to avoid problems and it is not only with other chopper stabilized amplifiers. You might want to synchronize with a sampling analog-to-digital converter, switching power supply, or microprocessor clock.
(1) In the past, high noise prevented chopper stabilized amplifiers from replacing precision bipolar parts even in low frequency applications. Maybe the OPA189 is different in medium impedance applications but I would sure want to empirically test it; the lack of a current noise graph and DC or 0.01 Hz noise specification makes me wonder if TI is hiding something.
I like the compound amplifier idea. This should make it possible to increase the bandwidth significantly? OPA189 might do a super job in a "supporting role"? I seem to remember seeing application notes with some fairly straightforward designs, but can't just recall where...
Usually it was about noise instead of bandwidth. Chopper stabilized amplifiers have flat flicker noise while linear parts have increasing flicker noise at lower frequencies and wide bandwidth parts can have astonishingly high flicker noise, but there is not much overlap between the requirements for low flicker noise and wide bandwidth except maybe in test instrumentation; I would not mind having an FFT signal analyzer with both but the common solution is just to use a separate low noise preamplifier.
That OPA189 is much better than past chopper stabilized parts I have used but total noise could still be improved by perhaps 4 times in a low impedance, 100s of ohms, application.