MFS vs MFJ is a whole different thing from MFJ vs 2x Single.

While there's numerous places where MFJ < 2x Single due to various marriage penalties, and even more numerous places where 2x Single < MFJ due to the shared tax bracket space below the 37% bracket....

there are very very few places where MFS > MFJ. They exist, but you have to be in particular situations like one person having disproportionately more debt and on an income based repayment plan, or similar.

There are of course many situations where 2x Single = MFJ = MFS if it's just two similar income W2 employees with no edge cases going on.

(Once you're married, filing 2x Single is obviously not an option)