Forgive me if this is a newb question, but are there any rules regarding 2+ wizards working together to create a spell and cast it? What about a ritual?
The spell will close a breach between the land of the dead and the living world. What kind of spell would it be? Perdo Vim? Creo Vim? 
See "Wizard's Communion" a Muto Vim spell on page 160 of the Core Book.
Welcome to the game!
No one part of rules is very complex (usually), but there can be a lot of diff rules sections. This is one reason these boards exist.
See "Help in the Lab", page 106.
Basically, one mage is lead, and uses her Lab Total. Other magi are the "helpers" (altho' there are other, more colorful terms, ahem), and add their Int+Magic Theory +/-Virtues/Flaws. For more than 1, the lead needs a high Leadership. But it's a social statement too, so IC the "helpers" need to be careful, and make sure they trust whoever is #1.
Create as above, cast as a team with Wizard's Communion (as G44 suggests).
So, there is a breach already existing? Urgh...
Outside standard Hermetic bounds, as that is crossing into the Spirit world, which is the domain of God. (Or maybe across the Lunar boundary? Something.)
But since the SG wants to go there, then I'd say PeVi, maybe with a Rego requisite to "carry" the spell where it needs to go (ask your SG). Look at ReVi "Opening the Intangible Tunnel" for inspiration.
You could use the PeVi General Guideline "Dispel effects of a specific type..." (2nd listed), if you first analyze it with a big InVi spell (another custom effect, most likely!).
If the SG wants to milk the plot, you might have to seek out non-Hermetic wisdom. (Um... which supplements have they been reading?) 
Scary. 8)
(If you have the Mysteries Revised, see page 100, new ReVi Guidelines, for more inspiration.)
Another thought is the realm magic breakthrough in RoP: Magic.
Of course there are more simple minded ways to seal the breach as well, such as rego terram.
Perhaps if we built a large wooden badger...
(I imagined the breech more "inter-planar", but if it's just a big door, then sure!)
Nah, more likely time for the Holy Handgrenade of Antioch.
Maybe if you gave it sharp and pointy teeth... No, wait thats for rabbits only i think...
A friendly warning about Wizards' Communion from what we experienced last night: each magus participating in the WC adds 1 botch die to the spell, and each suffers if the spell botches. Last night we had three of us in a communion. The magus casting the spell rolled a zero. Fortunately the aura was magical, so there were only 4 botch dice. He rolled a zero on the first. 
We watched in pain as he rolled each of the next three dice separately. Happily, no double-botch! 8) Of course, each of the magi picked up a warping point, but losing all three magi to temporary twilight simultaneously in the middle of an adventure would have been far worse.
Chris
great replies, thanks everyone! it's more of an inter-planar rift, not so much of a big hole in the ground.