Consider the following terms.  
• *[conjoined](https://en.wiktionary.org/wiki/conjoined#Adjective)*, “Joined together, as with conjoined twins, or in matrimony”  
• *[bonded](https://en.wiktionary.org/wiki/bond#Noun_2)*, using senses of *bond* such as “A physical connection which binds, a band; often plural”, “An emotional link, connection or union”, and “Any constraining or cementing force or material”  
• *[friendly](https://en.wiktionary.org/wiki/friendly#Adjective)*, reflecting that the two strings are quite alike  
• *[twined](https://en.wiktionary.org/wiki/twine#Verb)*, using the “to become mutually involved” sense of *twine*  
• *[woven](https://en.wiktionary.org/wiki/weave#Verb)*, using the “To unite by close connection or intermixture” sense of *weave*  
• *[mutual](https://en.wiktionary.org/wiki/mutual#Adjective)*, adj., “Possessed in common”; you would use it as a noun, after properly defining it:  “We call strings α and β *mutual* if...”