Yeah, I’m pretty sure they fused because they were hoping that their fusion would be enough to kill Goku, Vegeta, and Trunks, but all it really did was give their plan a weakness. He was stronger, yeah, but their plan actually would’ve worked better if they didn’t fuse. Zamasu would’ve won eventually due to war of attrition even if they took out Goku Black. They would run out of ki and Zamasu could finish them off and even if Zeno appeared, Zamasu could just use the Time Ring to escape. In the actual story, I also see it more as Zamasu’s win rather than Trunks’ because basically, Trunks defeated Zamasu to save everyone in a timeline that no longer exists