HELP Please: issue I/O routing with mono signals MOTU UltraLite Mk3

Hello all,
Something I need help with…
I have my Volca Kick and my Volca Bass outs going to 2 mono inputs on my Motu UltraLite Mk3:
Kick to IN 2 Left
Bass to IN 2 Right
The Motu outs through a D-Sub cable to MODEL1 mixer.
When either Kick or Bass channels are at lowest volume, I can hear the other one (very low but still can hear it) through the channel.
It seems the splitting of a stereo input in the Motu mixer is not absolute.
Anyone would care to chime in?
PS: channel 2 (ie IN 2 Left and IN 2 Right have been set up in MOTU mixer, considered as 2 mono inputs).