Edited to reflect conclusions from comments:
I want to construct a software RAID-like array with 2 storage devices plus one parity device, where the parity device maintains the XOR of the two other devices.
This is reminiscent of a RAID3 or RAID4, except that I'd like to be able to mount both underlying devices and write to them as I normally would, with the XOR being updated with each write to the devices. This also means that I do not want my data striped.
In summary, I want an arrangement which lets me freely store data two mounted devices, and a third device that has an up-to-date XOR of those two devices.
Standard Linux RAID4/mdadm will not work for me because it (1) stripes files across disks (2) prevents the mounting of individual disks.