I don't have a second SL so I can't check, if that works. But from what I know I can imagine that it is possible to cascade two SL's (possible only by firewire) and use a RM in I/O-mode (over AVB network). It would be interesting to see how 32+24 channels look in UC Surface and Capture. Capture should show 56+ tracks available, but UC Surface shows 8 more channels (57-64), that should come from somewhere, but where…?
In I/O-mode your RM32AI handles all the aux mixes on stage, which means that mixes 15+16 are fully available. But mains (left/right and mono) are "merged", they represent the mains of your SL 3242AI. But you can create another main mix on RM32AI, which is only accessable through phones out (mains faders on RM have no function).
SL and RM work as totally independend mixers, they only share the inputs. For the inputs you set as network inputs on your SL, gains, phantom power and polarity are handled on RM (remote controlled by SL and UC). An input on SL can also be network input for RM, which gives you quite some flexibility. If for example you want 4 signals from SL to be available on RM as monitor mixer, just set them as network inputs on your RM. Also talkback is available over AVB.
If cascading would not work by any reason, couldn't you just use any mixer and feed its mains into your SL 3242AI, There are two aux stereo inputs available with no fat channel but pan (from l/r over l-c-r to mono) and assign (subgroups, main). Unfortunately aux inputs A+B are no network inputs, but the tape input is…
Hope this helps.