Character becomes God, splits his godhood

[TomMazanec]

The book was back in the Seventies but set a few decades in its future (so around now). There were a couple test tube babies that had been partially successful, the protagonist may have been one. At the end he encounters God, who has gone insane from being the the only one of his kind. The protagonist somehow absorbs his godhood and, realizing what went wrong the first time, pours half his power into his girlfriend (I think) and they split the Earth between them to fight a war...the winner gets to create a new planet.