Any back up is way better than none!
certainly an identical back up would be perfect but I went a different route which is to use a less expensive model from the same manufacturer which takes the same mags and works in the same holster.
Main gun: CZC shadow custom (S1) , full custom job by CZC. Slide milled by CZC for carry optics.
back up: CZ 85C bought used, with a few after market parts installed by myself. Slide still milled by CZC. Way less expensive and still shoots extremely well.
I agree with Chuck that it is very nice for the back up to use the same mags and holster.