Your challenge is to have the Ghana Empire go Christian and the Kingdom of Kongo go Muslim, rather than the other way around as OTL.
There was contact (limited but still) between Kongo and the Swahili coast, though it was cross-continental by land rather than by sea.Meanwhile, the Arab Trade Route would have to be far more extensive past the Cape in order for the Muslims to trade with the Kongo tribes or the unified Kongo Kingdom and I don't know if they are willing to move past Kilwa if there weren't stories about riches past the Cape, so an earlier unification of Kongo might be needed to encourage trade from the East Coast w/ Arab and Swahili traders.
Your challenge is to have the Ghana Empire go Christian and the Kingdom of Kongo go Muslim, rather than the other way around as OTL.