Wow - lots of silly assumptions there... I know my Perdix AI and Teric inside and out. I also fully understand dive tables and how to use them.
I do 3-4 week long+ dive trips a year and don't ever want to have to cut a dive short or affect subsequent dives because I had a computer or transmitter failure (as unlikely as that actually is) - so I have full redundancy. Batteries are also cheap and easy to swap out, so I typically replace the batteries in my transmitters every other dive trip (and always carry spares). My Perdix AI has a battery level indicator as does the Teric - so easy to see when the Perdix needs replacement or the Teric recharged (Teric lasts several days but normally drop it on a QI pad every night on a trip anyhow). I'm not sure why you seem to have such an issue with this approach - it works perfectly for me!
To the OP - it's not a dumb idea and, if you have the means and deisire to do so, there is nothing wrong with a 2 transmitter (or even 2 computer) set up. Ignore the naysayers and equip yourself as you see fit in this area!
My issue is that just because it works for you at your level of compitance does not mean it is sound advice for others,
As haveing a spare comuter goes then I am somewhat like you however i leave my spare computer in the bag until i need it depending on the dive. Shallow dives you dont need one pter let alone 2.
Every time you change batteries you weaken the integrity of the area involving the battery. Whether you get a slight cross thread or get a spec of something on the oring etc. With a SPG you never have those issues. Generally if it is water tight on monday it will be 10 years form now also. I dont know much about the teric but i do have preditors and petrels. Shearwaters are made for the user to maintain unlike other brands. I am a die hard shearwater fan. That aside most can not handle one puter let alone 2. And many can not change from air to nitrox with out a book. Given that my comment was regarding computers in general and not specific the teric.
Most do not many know their puters inside and out and that is where first the problem starts and second they loose sight of what is needed. vs what complicates a problem. In most cases more is not better.
As to your comment to the OP,,,, If they can manage 2 comuters then fine. MOst people can not. Also as the rec world goes the puter is really nothing more than an electronic dive table, depth gage and watch. On a rec dive you dont need tables so a 1000 dollar backup is a pretty expensive depth gage and watch.