我的第一夫人你的第一夫人
[wŏ de dì yī fū rén nĭ de dì yī fū rén]
In this case, it humorously means both I and you have each other as our most important person. The term 'first lady' usually means wife of a president, but here it is used to describe the irreplaceability and uniqueness of their beloved ones respectively.