Why buy a backup bike? Why not buy a new bike and make the current one your backup?

But really, if you just want a bike for the trainer that sometimes gets ridden outside, don't even bother with mechanical Ultegra. Save some money and go with Tiagra or 105. You don't need electronic for a trainer bike.

(I do agree that it is very nice to have wheel and speed compatibility across bikes for easy swapping as needed.)