2 outputs in the Mult is no problem to my knowledge.
The Mult can be used to mix sound 2 outputs together, that’s what you did.
If it sound less dull, you probably touched the filter frequency, or the oscillator waveform.
Or the phase between oscillators might be different than before.
These synths are beasts, taming them is a dark occult art.
Don’t expect things to sound exactly the same two times in a row, let yourself go in the sound of the momentum and forget how it was before.
There’s actually a patch in the official Dark patch handbook that came with my Matriarch that has 3 oscillator Wave Outs going into a mult, and then that mix going a attenuator and then to one of the VCA ins. “Stereo Strings”. I can’t imagine they would suggest that if simply turning the attenuator knob too far up would damage something. Should be fine.
In general though I think outside of matched systems that can be a no-no for badly designed modules. I think for these semi-modulars it’s fine.