What assumptions does round earth require?
What assumptions does dual earth require?
Humour me, so we can objectively compare.
dual earth theory requires no explicit assumptions. we kow space exists, and the properties are logically deduced: those deductions are the only thing that could be called assumptions. they are a) less space means less distance (trivial) and b) it flows from high concentrations to low (a universal tendency observed in heat and pressure, among others).
they are logical, so i suppose the only assumption is that space behaves according to common, known laws.
round earth theory has gravity: which is not fully explained. there is an equation, a source, but no explanation for how that source produces the force. saying it can is an assumption.
round earth theory also has dark matter: equations don't match observations, so an assumption is thrown in.
there are many more minor aspects (auroras, which i've had a long discussion and embarassed round earthers over, the supposed reflectivity of the moon, etc), but those two major ones already make it worse than dual earth theory.