After 2 nights and probably 10+ hours attempting this Force Tear, I still haven't beat it. BUT I get pretty close and I'm pretty proud of this run.
Here's some things I've picked up on that may help you particularly on the Grand Master difficulty:
1) Get in some pokes and get out; greed will be punished by this Force Tear.
2) Avoid initiating bouts; your window for doing damage is in honing your reaction time and counterattacking right after bites (blockable) or lunging bites (unblockable). There's an audible mellow/high-pitched tone during unblockables that I believe indicates the perfect parry window (may be wrong on this though).
3) You can disregard 2) if you can keep your charged blaster bolts replenished, and create your own window of poking right after landing a charged bolt.
4) If you're blocking Oggdo, you're vulnerable to attacks by Spawn, and vice versa. Get good with Parries.
5) The standing bite (unblockable) can be parried by Parry Push.
6) RNG may set you up for failure if both bosses space apart an unblockable each where dodging/blocking/parrying one leads to your death by the second.
7) Creating some distance gives you time to telegraph the next attack(s) and react accordingly.
8) You can kite the double bites (blockable) by keeping just at the edge of one boss's reach, and swinging once after the second bite.
9) The tongue is not affected by Force Pull. I haven't managed to try swinging my saber at it yet, but I suspect its also not damage-able.
10) Play with your camera X-sensitivity as needed to keep both bosses in frame.
11) Avoid backflips as much as you can, as this adds a slight delay to your second jump that's enough to get you killed.
12) Flux Perk is invaluable for keeping a reserve of Force for an emergency Parry Push.