see configuration.h :
(can´t help more as i don´t have a corexy)
/* Define the type of axis movements needed for your printer. The typical case
is a full cartesian system where x, y and z moves are handled by separate motors.
0 = full cartesian system, xyz have separate motors.
1 = z axis + xy H-gantry (x_motor = x+y, y_motor = x-y)
2 = z axis + xy H-gantry (x_motor = x+y, y_motor = y-x)
3 = Delta printers (Rostock, Kossel, RostockMax, Cerberus, etc)
4 = Tuga printer (Scott-Russell mechanism)
5 = Bipod system (not implemented)
8 = y axis + xz H-gantry (x_motor = x+z, z_motor = x-z)
9 = y axis + xz H-gantry (x_motor = x+z, z_motor = z-x)
Cases 1, 2, 8 and 9 cover all needed xy and xz H gantry systems. If you get results mirrored etc. you can swap motor connections for x and y.
If a motor turns in the wrong direction change INVERT_X_DIR or INVERT_Y_DIR.
*/