Digital controllers can fail as well.
I was using a LifeGard digital monitor and alarm for a little over a year.
http://www.fosterandsmithaquatics.com/product/prod_display.cfm?c=3578+3744+3841&pcatid=3841
I had remote sensors in a number of tanks and used the monitor set my tank temps. Unfortunately it started giving very inaccurate readings after a year. I kept setting the heater dials lower until they bottomed out. It turned out that the actual tank temperatures were down to 70 even though the monitor was showing 81.
The best solution was already mentioned; use two low watt heaters.