Since the max mono channels available on the SL32 is 32, using the SL16R, as a submixer, seems like a natural combination of these two mixers to reach as many as 48 mono channels. 32 analog channels would go directly to the SL32. An additional 16 analog channels would go to the SL16R. These 16 would be separately mixed, then supplied via AVB to the SL32 Main L/R outputs. This would be done in manner similar to description in the SL32 Owners Manual, pg 74, section 22.214.171.124.
Using AVB communication is necessary to avoid using the Aux 1&2 plus tape inputs as these are already being used for music support.
Presently, such communication from the SL16R and the SL32 is not implemented. A strange restriction, for such an obvious solution to move beyond the 32 channel limit.