The Rasenrengan, is simply two Rasengan used simultaneously. Once created, both Rasengan's are used at the same time with greater force than a single, regular Rasengan. It is different than Twin Rasengan, because Twin Rasengan uses one Rasengan at a time to attack, while this technique uses both Rasengan at the same time. This, of course, requires quite a bit of chakra, likely double the amount used in a single Rasengan, which implies it might require Sage Mode.
Koga Tensei uses his version with the Nine-Tails chakra, granting him enough energy to preform this technique.