For what you want you need a disk server/linux cluster box of some kind, maybe with high availability raid.

Then, allow both servers to process read & write. When one fails the other is there by default. The disks are already there, mounted, served out by another box, so they do not have a failover problem.

This also solves the 'out of sync' problem.

This is almost exactly what everybody with a SAN is doing right now. Except for orders of magnitude lower cost. Our SAN costs us $US40 per MB; a PC with SATA can have a MB of raid for 2 orders of magnitude less. Circa $US1.00 - $US2.00 per MB.