I was actually doing it wrong and had the nerve to complain about itI was quickly pressing and releasing the two buttons instead of pressing and holding. Went from 100% failure to 100% success. Still wish they would implement something like the Zenfone 2 which uses a long press on the recent apps button.
I'd rather them do it like CM and long press on recent apps button switches you to your previous app. That makes multitasking so much quicker.