In an Eberron campaign I'm DMing two of my players are from House Thrask with the Mark of Finding, one is a Human and the other is a Half-Orc. The party calls them "The Twins." As they level up and get stronger I want to reward them with a buff to their marks, kind of like how UA has the "Lesser" and "Greater" dragonmarks. However because its a Mark of Finding and both Half-Orcs and humans can take the subrace option to have the Mark of Finding. I wanted to make the prerequisite be "The character must have the "Mark of Finding," I tried to include both subraces so it would be exclusive only to them, however it became unavailable to both, because you can't be both a human with the mark AND and Half-Orc with the mark. So is it possible to make the prerequisite be Human with the MoF OR Half-Orc with the MoF? Or should I make two separate classes feats, with one specifically for the human and one specifically for the half-orc?

You could just put the prerequisite in as plain text into the field. When you save it it will throw you an error code, but it will still save. The prereq won’t actually function mechanically in system however.