Tremere and additional Magical focuses

That's an interesting idea. While it is true that magi from another House could take MMF in Certamen, it is hard to imagine most players ever doing so. Certamen is so limited as a Focus that few players would be willing to restrict their growth in this way, unless, of course, that was the entire point. A player who picks MMF: Certamen is asking for a lot of Certamen in the game, and as a GM I'd like to give that player what he wants.

That said, I always try to discourage PCs taking virtues which another PC got for free; it really waters down the torture of Tytalus apprenticeship when another PC from a more friendly House has Self Confident. If I had a Tremere PC in my Saga, I would ask no other player to take that Focus.

In other words, while it's technically possible, I don't imagine I'll ever see a player outside of Tremere with this Focus. If it ever appears, it would be in an NPC made specifically to challenge a PC Tremere on his home ground.

Certamen as a focus only has value if you enforce the idea that the Order has a duellist culture.

Men in the southern parts of the US, and England, really did used to shoot each other to death in parks at dawn. It was serious stuff.

Certamen only works if you treat it as serious stuff.

Few people want to do that, because young PCs are bad at it.

So, basically it's a setting problem. We have a setting which says there's a duellist culture, but we don't use it for much in our games because a duellist culture seems unfair to PCs. I'd fix it at the setting level (by making Certamen victory the same as a War victory, socially) or I'd just get rid of Certamen altogether, because it's pointless.

It either matters or it doesn't, you know? Pick one and run with it, IMO.