I have trouble understanding this, perhaps you can explain it better? My reading is that you have two 12 volt fans, and if one fails, you want the other one to start up. Is that correct?
But how are you going to sense the fain failure. You seem to be saying you have a 12 volt relay coil is series with the fan, but that won't work, as both the fan and the relay will have less than 12 volts, they will divide the 12 volts between them.
And why a transistor switch if you have a relay?
You can sense the current into the fan via a series resistor, and an opamp or maybe a transistor can sense that and turn on the other fan. Not too difficult, email me.
But fan's don't fail in that mode, although I have little experience with fans. They tend to have a shorted winding and start to smoke, or perhaps get clogged with dust and have a restricted air flow and increased current.
You might want to consider sensing temperature and using that as the warning light.
High reliability fans and switchover are old technology. Did you research what has already been done in that field.