In the history of China's poetry, what works in what era does "Shuangbi" refer to?

Yuefu Double Walls refers to the best folk narrative poems in ancient China, Peacock Flying Southeast and Mulan Poetry. Peacock Flying Southeast is the first narrative poem in the history of China literature, which is also called Yuefu with Mulan's poem. Based on a marriage tragedy in Lujiang County (now Anhui Province) during the reign of Emperor Xian in the Eastern Han Dynasty. Originally a folk song, it may have been polished by later literati. Mulan Poetry is a long narrative folk song in northern China during the Southern and Northern Dynasties. It tells the story of Mulan disguised as a man, joined the army instead of her father, fought in the battlefield, returned to the DPRK in triumph, made meritorious deeds, and resigned and returned to China, which is full of legend.