Never mind whether Tremere can have another Focus; how does this one work? Focus usually doubles the lower Art in a Total but Certamen doesn't have a total. The most straightforward application seems to be doubling the lowest Art, which means that if you're fighting with Creo 7 and Auram 10, you have a higher effective score with the Creo (7x2=14). Nothing wrong with that, just feels a bit odd.
Alternatively someone might treat each roung in Certamen as a one-Art total, and double each Art...