Angier couldn’t trust himself, he couldn’t work with himself.
Angier felt guilt for his wife death, and wanted to know what it felt like to drown, while his clone took over to retain his legacy.
Or the machine was never working.
When angier gets shot and sees borden, he immediately says “a brother, a twin”. If the machine was working, and if borden was the one wo led angier to Tesla for the cloning machine, then angier’s first assumption would’ve been “a clone”, not “a twin”. This is further evidence that Tesla’s machine never worked.