Although the rules (as written and intended) explicitly disallow it (see Can a character have 2 types of Sorcery?) I would like to allow characters with more than one type of sorcery in my campaign.
It seems appropriate in the setting in my opinion, as magic seems to be hereditary and there is quite enough interbreeding among different nations.
So... is there any known reason for disallowing this? E.g., anything obviously broken or some known overpowered combination?
If everybody can only have one Sorcery, Sorcery types are free to distinguish themselves from each other with power and advancement mechanics without worrying about the consequences of somebody going half-and-half. Some can start weak and get stronger, some can start narrow and get wider, some can be a little bit of both. When you introduce the idea of one person with multiple sorceries into the game, you're not only making sorceries compete for bought advancements when they were never meant to, you're opening several related cans of worms:
Not all sorceries advance or max out evenly. You can get most of Sorte in three advancements; the fourth is just to upgrade your last minor power. Three advancements is what a Knight of Avalon would call "a good start". It's certainly possible for a half-blood sorceror to reach the peak of power in one of their national sorceries, and then just keep going in the other one.
Some sorceries come with a plot hook. And some of those plot hooks are kind of a big deal! It's okay for a 7th Sea player character to be kind of a big deal, but it starts to strain a bit when you've got noble Ussuran blood, and Grandmother Winter thinks you're worth teaching a few lessons to, and, simultaneously, the sidhe judged you worthy of picking up one of the twenty metaphorical swords of the Knights of Avalon.
The personal power and price of sorceries is not set down with the expectation they might combine. The necrotic alchemy of Hexenwork produces powerful physical things you need to interact with; if one of those physical things lets you Wraith Walk and leave your body behind, you're ghosting around at the cost of being cut off from the rest of your sorcerous powers. If you somehow had access to the dievas deals of Sanderis as a ghost, though, nothing would stop you from invoking them to cause demon chaos while your body rested somewhere else in safety. Or, alternately, Sorte reaches full power with a fairly low point investment in part because its use demands a price, in blood or otherwise, but that blood price loses a lot of its sting if you've also got access to Wanuy Naqay and you can Leech it right back off a dead man's blood. (Speaking of Wanuy Naqay, its powers are invested in Death Tokens, precious physical objects you may have to lose and compensate for. What if you could just put a Minor Porte Mark on them?)
Sorceries aren't meant to compete with each other for advancement, or do an end run around their own power caps, or collide with each others' stories, or build off each others' strengths, or shore up each others' weaknesses. If you introduce the ability to freely pick among multiple sorceries you're going to be running into all these wrinkles.
