End on a status quo.
I suggest your book should end on a status quo (land the plane). Do not begin a new conflict that will go unresolved.
A logical place to 'land the plane' is to end with the character having been kidnapped, and the sequel tease that the forbidden realm is now discoverable – maybe, thanks to the 'new' thing. There is a tease for the story to continue, but those teases are not yet exploited for story. Instead they must function as an ending for the battle that just happened.
The 'loss' of the character is a new status quo. They can't reach her, and she may be dead –– or worse. This is important character-building, emotional stuff for your team that you do not want to gloss over.
Give them the pyrrhic victory and let them grieve. Also let your readers grieve the loss of a main character. Make it count.
Begin on a shaky status quo.
The story should resume in the next book preparing to move on from an un-settled status quo –– not necessarily the same status quo from the ending of the previous book. It's natural to imply a hiatus between books We skip over time in the hospital, and ship repairs. Progress has been made on the 'artifact'. They are smarter now, they know what they are dealing with.
But this status quo is unstable, there is aleady a conflict here. There is urgency to go on the attack, but all those battle allies are settling back into their lives. No one really wants to go to another realm, not for someone who isn't alive. The heroes have needs that aren't going to be easy to get.
The reader meets these characters anew in their new shaky status quo. Team roles will have shifted without the missing person. Example: the missing group member is 'The Heart' of the team who mitigates conflicts between the team leader and 'The Lancer' (the #2 guy). Without her, they can't resolve their arguments. The team is currently dysfunctional and not capable of fighting a battle in another realm.
Cliff hangers destroy tension.
If you transport everyone to the other realm, and they run into their mirror-universe friend right away, there is no space to develop any of this internal team conflict, there are no stakes to getting the team back together, or to rescuing their friend.
Think of every cliché cliffhanger: Polly is tied to the railroad tracks as a steaming locomotive AND an airplane crash on her, and a bomb explodes. Polly is surely dead this ti– nope, there was a trap door and a magic ring and some deus ex machina plot-armor....
She's fine. She is always fine.
Cliffhangers are terrible narrative devices because they withhold the climax of the story. It's all build-up, no pay-off. Any 'tension' the audience experiences between episodes is not anything you can capitalize on within your story.
What should be the climax is now over-shot, like jumping the shark, it's too big. Picking it up again… whenever, later… is going to be anti-climactic no matter what. You've lost any 'tension' that might have been there, and now you have to walk-it-back to reality.
Readers know how this works: no corpse = not really dead. That is what the team is saying too, so readers and team are on the same page. It's ok because the emotions to milk here are better than 'surprise', 'twist', 'wtf'. You can build up hope, and then sew doubt. You have main characters under a new kind of stress, and they can express it by falling into less heroic versions of themselves: angry, self-harming, headstrong, hero-egos.
(Chewbacca was in a different shuttle. Did you think he was really dead for 42 seconds? Nail-biting tension. Don't blink or you'll miss the character growth.)
My suggestion is to withhold the missing team member well into the 2nd book. Make the team struggle to find her, sacrifice to get to her. Make them fight every step of the way, growing more resentful and dysfunctional. Crossing the line.
And then they discover once they are stripped to their lowest morale, that... she's fine – just a little evil. Reader is now intimate with the teams weaknesses and dysfunction, it will be delicious to watch Evil Friend exploit each one of them.
Where to cut
You can hint at what's to come, you should probably always be hinting at what's to come, but keep the conflict and pay-off within the present story.
Each book should stand complete with a satisfying internal conflict arc – that is the narrative reason to have multiple books (not to take up more bookshelf with planned sequels, lol).
Dividing a book may require some re-structuring to clean up the overlap. Start and end on a status quo, and reset the heroes to 'zeroes' in the sequel so they are forced to begin a new character-growth arc they are uncomfortable with.
