It's simple, if they're not the same algorithm you follow the more conservative one. (I don't think you should even push the more conservative one to it's NDL.)
I get tired of reading statements that there's no benefit to a backup computer. I mostly dove tables when I was doing local diving, and it was perfectly adequate for what I was doing. 2 computers is appropriate for the type of diving I do now. Everyones diving is not the same. It's been covered on other threads, but - if I've paid a lot of money to get to an distant place and plan to dive 4-5 a day, and one computer craps out, I don't have to call a dive or sit out anything. Especially when the next dive up is that world famous site you've been waiting to see for 10 years. There's a LOT of value to that. Sure I could keep track and see what the tables say, but given the typical profiles in this type of diving the tables will generally declare me to be dead so that's of rather limited use.
Other people will have other reasons for wanting a backup. Maybe they're doing a couple dives locally on the weekends and it's not as big a deal to bag and sit out, or falling back to the tables is more viable - but maybe they still don't want to do that. If they can afford the second computer why the heck not?