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Weapon+Shield is indeed accomplished by adding the combat statistics of both weapons. The only instance of a dual-wielding characters in 5E canon is a Mamluk Emir in Realms of Power: The Divine. His stats don't follow the above rule, however, instead he appears to use the Single Weapon skill with the stats for a single longsword but with +1 added to each of: Init, Atk, Def, Dam, and Load. As said though, I basically had to reverse engineer the character and it's possible Niall had a more nuanced system in mind.
Edit: The aforementioned Mameluke uses two longswords, but if a character is to use two weapons of different sorts (dagger and axe, for example), I would recommend that the using the better stat for the better weapon +1 when determining Atk, Def, etc. -
I've never played it that way and, rereading the relevant section on page 178, it sounds like you roll once after the combat is over.
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As previously mentioned, Art & Acadame has a more detailed method of treating disease and injury as well as an overview of how medieval scholars viewed the functioning of the human mind. Ultimately, it's a book about life in Mythic universities however. Realms of Power: The Infernal, and Against the Dark: Transylvania Tribunal might also be good options for running darker stories than are typical of Ars Magica.