I had the identical problem on my Land Cruiser. I replaced only the one bad sensor. Bank 1 is the side that does include cylinder 1 (I believe this is the front side). Still no failure on the other one, a year later.
I bought a Bosch sensor at Pep Boys for about $75. I have also seen these on eBay. If you buy the Bosch, don't be afraid of using their special connector.
Pep Boys also loaned me the special socket I needed to force it out and reinstall it.
Be sure to use anti-sieze on the new sensor.
These O2 sensors have a heating filament in them to get 'em working faster than normal exhaust gas temp. The filament is not too much different than the filament in a light bulb, so it's not unusual for one to fail and the other one not to.
Mike