[zzhk]

I think it's a valid question, though "demi-god" needs to be clarified first.

As far as we know, Heretic Gods don't bear children with humans, so there aren't any demi-god offspring born from such a union. Subordinate gods like Ama no Murakumo no Tsurugi are not human to begin with, while Ena in divine possession could be considered in a semi-divine state but she's still human.

So actual demi-gods becoming Campiones is a non-issue, but it doesn't rule out characters known as demi-gods in legend from being actual Campiones in history.

It's entirely plausible for an ancient Campione to slay multiple gods and gain their authorities, thereby engendering new legends of a god-like being with elements from his progenitors. That could be one way for seemingly disparate deities to suddenly get associated with one another.