I wanted to ask if we have intercourse during periods, how do I perform ghusl? Do I have to perform 2 ghusl separate or just 1 with niyat of both would be enough?
Also, I know this is forbidden in our religion, what is the kuffarah for this?

It’s sin. you have to ask forgiveness to Allah for that. There is no Kaffara wajib for that. One ghusl with two niyyat is enough