I've already posted basic multipath support for qemu last week.
However, this still relies on the non-obvious hack to specify the same file for both block devices, thereby implictly doing multipathing. And it has the drawback that one can only use the 'raw' format for images, everything else won't work.
So this project is about implementing real multipath support for qemu, where one blockdevice is specified which serves as a backing device for two (or several) SCSI disks. It also means one has to break the current object model, as currently the block devices are embedded in the SCSI disk structure, which obviously won't work any longer.
This project is one of its kind!