Hi Shaun
All meters have to comply with the same accuracy standards of + / - 15% so as you can see they are not as accurate as people think. The problem with using 2 meters is you have no idea which one is the closest to your real glucose levels. It's possible one could read 15% high & the other 15% low, the manufacturer would consider both to be acceptable. The fact is you could take another test with a new strip in the same meter and still get a different results. The only way to get a truly accurate blood glucose level is to have the sample of blood tested in a lab.
I suggest your mother just uses the one she likes best and keeps the other as a spare