Xiaomi A2 has the same hardware as Mi 6X and actually, Mi 6X is the Chinese version of A2. Why not fully flashing Mi 6X with A2 ROM, including GPT structures from the firmware? The problem will be fastboot itself, as you can't write on GPT there or change the name of the partitions as it brokes the existing fastboot in the current ABL (Android BootLoader), but it should work if ABL itself is changed to the one in A2.
You will need to use EDL mode to fully write A2 ROM on Mi 6X with A2's partition table, or at least write both GPT structure and ABL.elf with other bootloader partitions like XBL.elf and then use fastboot (which will be flashed from A2 so it must recognize and respond to the name of the partitions from A2) to flash the remaining partitions.
This should be possible only if both devices are typically the same in hardware and SoC firmware and there isn't Secure Boot eFuse provided by Qualcomm in the Mi 6X SoC, as this will prevent a modified or different ABL (Android BootLoader) from being loaded in the first place (even if the bootloader is unlocked) which will bring you again to EDL mode with boot error. So first, check if you have secure boot enabled or not by something like
fastboot oem SecureBoot GetFuseStatus if your device supports this command, or by directly flashing different A2 ROM with its GPT structures and see what will happen.
You should also know that what you do is risky and you have a pretty high chance of bricking your phone, so do it at your own risk.