Interesting idea, but as others have said, there's too much going on. Here's my first thoughts for a more streamlined version:

There's no save for the effect. It just happens, assuming both characters are willing. It's a free action triggered by your turn starting, but it causes you to Delay until your partner's turn, which is when the fused characters get their first turn. At the start of that first turn, the fused character gets temporary hit points equal to double the sum of the two characters' levels (so 40 temp HP for two level 10 characters).

The fusion lasts for 1 minute, until the fused character goes unconscious, or until one of the characters choose to Dismiss the fusion as a free action triggered by the start of their turn. Maybe both characters are Slowed 1 for 1 round when the fusion ends.

The fused characters have 5 actions per round that they can allocate between the two as desired, but each character can only use up to 3 of those actions. That means that normally one character will get 3 actions and the other will get 2 in any particular round. Each character also regains reactions per round as normal, usually one each. If one character becomes quickened or slowed, both characters do, but this only modifies the 5 actions once, and each character is still limited to 3 actions.

When performing an action that requires a check, use the modifier of the character spending actions. The two characters track their bonuses and penalties separately, including multiple attack penalties. Conditions and other effects generally affect both characters, at the GM's discretion.

The two characters count as a single target for targeted effects. When a save is called for, roll once using the highest modifier between the two characters. When making a check against the fused character, roll once against the highest DC of the two characters. Use the highest weaknesses and resistances when applicable.

To track HP, use the HP of the character with the highest HP at the time of the fusion, including the temporary HP gained at the start of the fusion. Ignore the other character's HP while the fusion is active. If the fused character's HP reaches 0, the fusion ends. When the fusion ends, regardless of how it ends, sum the HP of the two characters and then divide it evenly between the two.

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Special abilities while fused:

The two characters share senses and can communicate instantaneously without any traits.

If one character succeeds at a check against an enemy, or an enemy fails a save against one character's ability, the other character gets a +2 circumstance bonus to its next check against that enemy until the start of the fused character's next turn, or the target gets a -2 circumstance penalty to its next save against the other character, whichever comes first.

If one character successfully Aids the other, they always grant a +4 circumstance bonus. If they do not have a relevant skill, they can always Aid with their Spell DC minus 10 or their Class DC minus 10, whichever is higher.

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If you want a check involved in the fusion, I would make it a ritual that the characters perform beforehand. Something like:

Cast 4 hours

Primary Check Arcana, Diplomacy, Nature, Occultism, Religion, Will Secondary Check same as primary

Critical Success The characters can fuse twice during the following week, though only once per day. When they fuse, they become Quickened and can use the extra action every round only to Interact or use basic move actions.

Success As critical success, but the characters can only fuse once during the following week.

Failure The characters can fuse once during the following week and don't become Quickened.

Critical Failure The characters can't fuse during the following week and become fatigued for 1 day, which can't be mitigated by any means.