Besides Superman going "OMGZ, you're really powerful, Billy", their fusion so-to-speak didn't really show anything that would put them at or beyond Thanos' level.
Superman and Billy are about equal. Superman has beaten Darkseid. Darkseid and Thanos are bout equal. Super Marvel has double the power. Super Marvel wins.
You took a hamburger and added cheese.
Thanos eats them
__________________ Quotes from Hia8:
"I claimed that the science is sometimes faulty."
"You don't understand. This is fiction. That means none of this stuff really happened."
"There is no writer to purposely ignore a character's natural ability just because it suits the story."
"in some cases because the writer knows that Character A will dominate Character B easily and refuses to allow this to happen for the sake of the story."