Can I copy or move apps between iPhones?

There's an app (Ryobi's PhoneWorks) which I have on my phone but can't run because isn't compatible with any recent OS. I do have an old iPhone which is so old that the OS can't be updated and would be able to run the app - but it's not on the app store so can't be downloaded.

Is there any way to copy or more it from one phone to the other?