Euphemistic literary term for breasts or buttocks, especially in erotic writing; literally means 'twin hills'. The literal sense is rare in modern standalone use.
In that erotic novel, the woman's breasts were described poetically.
「双丘」は文字通りには二つの丘を指すが、現代ではほぼ使われない。
'双丘' literally means two hills, but is hardly used that way in modern Japanese.