Xecure
The problem resides in using encrypted partitions on MBR and not UEFI. All this information you didn’t initially provide changes the answer completely.
On UEFI, boot .EFI files for each boot partition are saved on a separate (EFI) partition outside the (encrypted) partitions, so a single boot partition can find them easily. If you want dual boot on MBR, the simple solution I can come to (based on other topics, an unreliable solution) is for you to have two boot partitions, one for each encrypted partition, and only use one for MBR.
The other solution is to mount the other encrypted partition from the installed system and run sudo update-grub (based on one of these answers)
It is best to ask in the MX forum, as here you can only find unreliable answers, based on another poster.
antiX Live system enthusiast.
General Live Boot Parameters for antiX.