which is infinite at final form?
An asspull?
Well the sage was strong enough to defeat the juubi, so it would make sense he could be strong enough to split its chakra.
Also the juubi doesnt have infinite chakra. Its just the sheer amount of chakra it had was so vast that it was thought to be infinite.
(kurama was said to have infinite chakra as well, and according to gyuki all tb have the same amount of chakra,soooo)
basically the juubi has at least 9x near infinity chakra.
basically the juubi has so much chakra, that the easiest way to comprehend it is to compare it to being infinite.