Is there an example in canon of two (or more) magi sharing, not merely a lab, but a sanctum? Legally, each would be giving up their Code-guaranteed protections from the other(s), so trust would (I presume) be exceedingly high. Is there a Tribunal ruling in canon on such a thing having happened?
No example that I'm aware of.
Consider that if there is a Peripheral Code ruling on such a thing, then it was an issue that was brought before Tribunal and adjudicated. So, if this is something that some in your troupe are exploring you need to determine if you're the first to try it out. If you're not the first, what happened the first time that caused it to become a ruling enshrined in a Tribunal's Peripheral Code?
I can only imagine such a happenstance for two married magi, and even then, I think they might have one sanctum and two labs within that sanctum... Andru Jerbitonis and Carmine Flambonis are married (two canon NPCs from Sanctuary of Ice). Outside their shared sanctum they have a two sancutm markers circumscribed by a circle denoting that this is a shared sanctum for the two of them? I dunno. This wasn't mentioned in Sanctuary of Ice.
Sundered Eagle has a pair of gifted conjoined twins, although a magus might argue that this is an exceptional situation.
IMS, I'd allow a married or paired couple to use the same sancta, but If it ever came up I may have an angry mage who got toasted by the spouse try to raise it in tribunal. I doubt it'd go through.. but since when has Tribunal not been about factional politics despite common sense?