Interestingly enough this is when you debug using --multikey:
"(Scheme function)"
m:0x8 + c:108
Alt + Alt_R
"(Scheme function)"
m:0x8 + c:64
Alt + Alt_L
Which is not the same with RControl
"(Scheme function)"
m:0xc + c:105
Control+Alt + Control_R
"(Scheme function)"
m:0x8 + c:64
Alt + Alt_L
I guess something particular on xbindkeys just does not allow for both alts to be inputted synchronously. I will just use Alt + CtrlR as a compromise, since what I wanted is incompatible.
No clue as to why Pressing only AltLeft gets fully overridden.