Joined or paired together; connected as twins or in a twinned relationship.
From cotwin + -ed, or possibly from co- + twinned (past participle of twin, meaning to pair or couple). The -ed suffix marks it as an adjective describing a state of being paired.
Cotwinned is so rare that it might only appear in historical crystallography texts describing minerals that grew together—it's a beautiful example of how technical fields create super-specific vocabulary for precise phenomena.
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