This is a very fun idea. Here are some of my thoughts, which you hopefully find useful. Feel free to use one or more ideas, or none if none suit your campaign.
Your Co-GM's Character Has Other Things To Do
You may both know the major things that affect the world, but your characters have much more immediate things they have to take care of; and that neither of you have to share with the other.
Without knowing details about your homebrew world, here are some examples that illustrate what I mean:
If the next major event is an insurrection of the people against their tyrannous vampiric oppressors, one party might be a pocket resistance smuggling weapons and warfare paraphernalia; the other party might be a collection of people, meeting for the first time on the ferry away from the city, trying to escape on the eve of the rebellion.
If the next major event is the invasion of the villain and their army, one party might be in the thick of things, present at the moment the walls were breached and witness to the fall of their kingdom; the other party might be returning to the city after the scourge has passed, coming home to ash, dust, and rubble.
If the next major event is the return of a magical winter, one party might be busy paying absolutely no attention to it while fully knowing it's coming; the other party might be busy telling everyone they meet that it is coming.
The point is, just because you have a "shared destiny" in the world, doesn't mean you will experience it the same way, or experience the same things along the way.
Go for Simplicity
Don't decide over too many plot details with your co-GM. Go for simplicity.
Instead, set up something like "prompts" -- a small phrase that fully describes the entire core of the next "major event."
That isn't to say don't flesh out the world with him; you still do that. This doesn't mean don't agree on how the villain will ruin everything. It also doesn't mean don't spend hours designing the campaign with him.
The idea is only to set up a string of "prompts" to form the backbone of your metaplot, that roughly describe (1) what is happening, (2) where each party is, and (3) why they care about what is happening.
Design The "Major" Events Well
Imagine scenarios where, knowing what's coming ahead, when that moment arrives, each party experiences it in starkly contrasting ways. The goal is either to put in irony, serendipity, or revalation -- or something else that feels awesome.
On the King's coronation day, one party is standing beside the King; the other is about to shoot him in the head (and get away with it)
On the day of the villain's prison break, one party is summoned to give chase; the other, oblivious, is looking at a list of jobs at a tavern when a cloaked, hooded figure speeds past them on a horse, pursued by a party of mercenaries
One party gets a quest from an NPC; the other party comes across the same NPC later and is revealed to be a demon
But once you've decided roughly what's going to happen next, stop there. Put an emphasis on the fact that the next "major event" affects the two parties together very tangentially, but affects the environment a lot, so both sides have a lot of room to be affected by and interpret the circumstance independently of each other.
Forget The Metaplot
Create an independent, self-sufficient story arc independently from your overarching plot. Then, at times that your private story arc interacts with the metaplot (during these "major events"), flesh out how those two lines clash and merge (but only by yourself, your co-GM doesn't need to know).
This is a good way to catch your co-GM's engagement in your story, and keep them on their toes. Because you're GMing as if there was no metaplot, they will have no way to use their knowledge to benefit themselves, even accidentally, unless they try really, really hard to break the campaign.
The Party Will Surprise You Both
It goes without saying, but you may not have to keep your co-DM on his toes yourself, because you yourself will be constantly adjusting to your players.