Hi Hugo,
you are right, MQX does not support second core e200z0. And it is not possible to modify the MQX to support the second core. The only way how to run MQX on both cores is to use two independent MQX images.
Dual core support has been implemented only on MPC5675K because there are two identical e200z7 cores. Only single MQX image was used and there were two RAM areas for each core. MMU unit was used to re-map the RAM virtual addresses, so data area for each core was virtually placed at the same RAM address (this is important if you want to use single MQX flash image).
e200z0 does not have MMU, so it is not possible to implemented it in the same way on MPC5668G.
Regards,
Lukas