Semi-joke suggestion:
The “double-ender”. A double-ended expansion card intended to join two framework laptops at the hip.
It would have different modes:
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Hardware KVM switch mode. Whichever laptop is “active” gets control of all peripherals of the other including screen, keyboard, other expansion cards, etc. so you can use it to get dual displays.
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Software KVM switch mode (think synergy). Mouse can be moved from one screen to the other, files copied across, etc. but each laptop still controls its own hardware.
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P2P mode. Simply networks the two laptops together.
You could also have a variation where they’re connected by a flexible cable rather than fixed in place to allow the two laptops to be used independently (gameboy link cable…).