结婚后双方账户必须合并吗, that's evidence for USCIS to grant a family based

Marital assets include all income earned during marriage notwithstanding you deposit into a joint acount or not, but exclude inheritance, gifts.

Assets acquired before marriage, if you keep it separate and maitain its nature, it may still be separate assets.

A joint account is only a form of two people's union. Substance is over form. If you are not sure, consult a matrimonial attorney on how to protect yourself financially.