Slightly OT: stereo and mono audio splitting

[Tenacious]

I would think that you need to home-brew some single amplifier stages to do what you want.  If each Amiga audio channel feeds the 1084S and a simple audio amp made from a 2n2222 or 741 op-amp, then the outputs of the amps could be combined into mono without affecting the inputs that also feed the 1084S.  

There could be loading issues of the original Amiga audio outputs if the new amps and the 1084S have different input impedances.  This might need 4 identical  amplifier stages, 2 in parallel for each Amiga output.  One from each channel would feed the 1084S, and one from each to be combined into mono.  There are many quad op-amp packages (LM348, etc) that might do this nicely with a few supporting components. Don't forget the de-coupling caps.
(To be read fast in an annoying, corporate voice.)  This is merely speculation on my part, I haven't done this myself.  No warranty is expressed or implied.  Your mileage may vary from the figures listed here.

Grin.

[Tenacious]

@ ral-clan

It's a good thought, especially for a DC signal.  Unfortunately, audio signals usually swing both ways, positive and negative about the referencing ground.  A diode would 'clip' half the information.  

The resistors might give passing results.  The stereo sound might have noticeably less seperation in the 1084S, and, the mono signal may need more gain (higher volume setting). It's easy enought to try.