Have y-axis movement result in negative x-axis movement in marlin

Hello All,

The problem: Inspired by the new generation of dual extruder printers and the option in marlin enabling the use two independent x-axis extruders, i started working on a new concept. For me it started BCN3D Sigma which has two Independent moving extruders. For me the biggest disadvantage is the 2 stepper motors hanging on the y-axis. This really increases the weight and therefore decreases the dynamic properties and fast print quality. Also i personally like the direct extruders more, since 1.75mm filament is more widely available and i already have a lot in stock. This would mean 4 stepper motors are on the y-axis, resulting in a very heavy weight structure

Solution: To solve this problem i came up with a concept sort of based on corexy. I attached a picture of the concept with the 3 different belts running in all directions. The only problem with this concept is that moving the y-axis and holding the 2 stepper motors for the x-axis still, results in movement of the extruder carriages. Moving the y-axis up moves the extruders to the right, and down to the left.

Where i need help:Marlin needs to be adjusted so a certain distance in y-axis (from the g-code) is subtracted from both the x-axis steppers. So also from the inactive one. Or somebody can tell me how this can be done in the g-code. My first thought was to make a post-processing script subtracting the y-axis movement from the x-axis, but this would only solve the problem for the active extruders and not for the "parked" one. Also i would like to solve this problem in the software to keep the structure as simple as possible and because i really think is possible to do. Who could help me with this? I would really appreciate this. Also i have no problem sharing the concept, this is purely hobby.

With kind regards