what I can remember it is about a male character who finds out he was a female in his last life and can see spirits. also his intended was a male immortal spirit of some kind, I think there was only 2 books in all. but I'm not sure. 🤢

It sounds a little like Tokage.

There are 2 male MCs. One is an immortal spirit (formerly male) "reincarnated" in a female body. The other IIRC can see spirits (I remember him noticing something strange about the dad/girl that meets them). And later in the story it's explained that he was formerly a female, who was "married" to a god (your immortal spirit ?) and intended to be sacrificed, but things went wrong.

Shuto Heru ?

Bokura no Kiseki?
NG Life?

Can't say much but its not NG Life for sure, since he(the protagonist) is not a female in the past. Even if the starter is talking about the non-main character, none of the characters can see spirits.