If you'll recall, Raikage failed to predict where Minato was going to teleport to, and it is far from predictable when the user can place additional seals any time he chooses quite discretely. Obito was about to use the human path on one of Danzo's body guards earlier in the series, and he was using the six paths technique with the edo-jins, and madara being the original owner of the eye should be able to do at least what obito can do with a single eye, granted that didn't display all of the rinnegan abilities it displayed some of them, as opposed to none at all as it stands with Sasuke. True it was stated that both eyes unlock the full power, that could easily be referring to the magnitude of the power of the techniques, not the techniques themselves. In Sasuke's case, it is very doubtful he will get a second eye, I think whatever power he has now is all he will have, and if he has anything else it's a matter of him not showing it yet, as opposed to not having access to it.