"Guitar –> input 1 output 1/ –> Amplifier 1
instr level>Space>amp level
Synth –>/ input 2 output 2 –> Amplifier 2"
This should work fine, set the Space input level switch to instrument to match guitar output and adjust the synth output down to not overload the fx input, some synths have -10dB outs. Some effects algorithms particularly DualVerb and the dual mono type delays will stay very separate, some algos will wildly comingle the two channels with modulation, panning, reverb, etc.