I like the back up camera to be the same as my main camera, I duplicate the settings in both.
That way, the quality of the files are the same, I know no matter which one I pick up, I don't have to think about it. Not layout, not cropped factor, not ergonomics or even things like both taking the same kind of cards.
I back up my lenses by having a set of primes and also zooms for the same focal lengths. Just get a 16-35 and 24-70 would be the zooms and then a set of primes. I know you said you don't want to spend $$$ but honestly, you are doing yourself a dis-service by not doing so. Invest in yourself and your business and it will reward you in the long term.