It's unlikely that you'd get a simultaneous tie; you'd expect one computer to go bad before the other. But I think in that case, the astronauts switch to the Backup Flight System, the fifth computer.

Mission STS-9 had two computer failures, causing landing to be delayed by 7 3/4 hours. They carried a sixth computer as a backup for following missions.

As far as how the voting works, each computer has a signal indicating what it thinks the status is of each computer, including itself. (Computers can detect many failures from self-checking, such as parity errors.) Each IOP uses these votes to determine the "redundant set", calculating the votes in hardware. The status is also displayed to the astronauts in a 5×5 grid. Astronauts can power down a computer or reboot it.