In regards to a session with only one of the necessary aspects: there IS a possibility of a session with only a space player succeeding. The time player is needed in case a scratch is necessary, whereas the space player is needed to create the new universe. If by some chance a group were to do everything perfectly, a time player might not be needed to have a successful session.

Both a Time and Space player are needed because those aspects lay the foundation for any universe. If a universe was somehow made lacking one of the aspects it would be fundamentally flawed and probably wouldn’t be able to create another universe.