It was probably just that good ol' Goku willpower. It doesn't really explain how he could use Blue, use a Kamehameha, get his ass beat, get pelted by extinction blasts, then go Blue again with any hope of winning. This seemed to be borrowing heavily from the Pure Boo portion of the Z manga without thinking too much about power consistency. At least in the Z manga, Pure Boo wasn't the strongest fusion ever, though. What's going on with Zamasu here is like if Goku and Vegeta wanted to fight Evil Boo separately, despite not being much stronger than Fat Boo (as an example). It's stubbornness and stupidity.
Gowasu's line about hoping Goku could last A WHOLE FUCKING HOUR just makes it so much worse. He should know right away that shit's impossible. Goku wasn't even stronger than regular Black, let alone a fucking fusion of the guy.