it will most likely be both of them to end it using there signatiture base moves .. sasuke will probally use chidori and naruto will probally use base rasengan and then will do it at the same time :|
I think both of them doing it will be like they did with Obito so... Madara in my opnion will probably kill himself with power or the Juubi will reap his body like it almost did with Obito.