I just stumbled across this. I have a fair amount of experience using mixing hot ends and I doubt what you want to do is possible (but it might be). The reason being that a G1 extruder move is always in one direction. You could possibly reverse the overall direction by using absolute extruder moves, rather than relative or by using G10 retract. But all that happens with a mixing hot end is that each extruder move is a proportion of the whole move, depending on the mixing ratio. But only positive percentage values (usually 1 - 100) are allowed (I think). What you would need is to be able to have a mixing ratio of say -20:+120. That would give movement of minus 20% for one extruder and plus 120% for the other. I doubt very much if the firmware would allow that but if you could hack the firmware to accept negative values, it might be doable. Or maybe there is no such limit in the firmware - try it and see.
Another option might be to define the tracks as axes rather than extruders but use relative moves rather than absolute. We usually use G90 so that all coordinates from this command onwards are absolute, relative to the origin of the machine. But if you sent G91 instead of G90, then all coordinates from this command onwards are relative to the last position. It's not infinite but should allow you to do pretty big moves without being constrained by the build platform dimensions. So then you could send say G1 X100 Y-50 or some such.
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