Live switching AC loads with TRIACs or SSR
Sounds like you are trying to build an Automatic Power Switch. Since increasing reliability is the motivation here, it might make sense to purchase a commercial product that has already been field tested. It is likely that a commercial solution will not only be more reliable than a homebrew one, it will also likely be cheaper as well once you factor in all the time and effort.

Take a look at some of the models from Tripp-Lite, which are used in data centers...

http://www.tripplite.com/product/automatic-transfer-switch-pdus/942


  Automatic Transfer Switch PDUs Provide redundant power for
  single-corded network devices with PDUs with Automatic Transfer
  Switching (ATS)
  
  Dual ATS input cords support connection to separate primary and
  secondary power sources If the primary power source becomes unstable
  or fails altogether, the ATS will switch to the secondary power source
  until the primary input is restored and stable ATS functionality is
  supported by any two compatible AC power sources, regardless of phase,
  to support a variety of advanced redundant power networking
  applications Models with the “NET” suffix offer switched PDU outlets
  and built-in network interface for remote monitoring and control