Measuring a high dynamic range current decay with different amplifier gains
Several things pop to mind:

Match the bandwidth of the two amplifiers.  Without that, you'll have a hard time doing a good overlap.

Adjust the relative gains of the two readings depending on the high gain signal.  Above some limit, the output comes totally from the low gain signal (Gl = 1, Gh = 0).  In a transition region near the top of the high gain signal, linearly blend.  Below that, just use the high gain signal (Gl = 0, Gh = 1).

Calibrate the relation of the two amps at a few points with a static DC signal.  You would then apply this calibration to one or both signals before the blending.