The traditional 雙 held two birds 雔 in one hand 又; simplification kept just the pair of hands. Two of the same thing, working together: 双 is the character form of symmetry.
双 is auspicious arithmetic: 好事成双 'good things come in pairs' governs gift-giving (never give one of anything), and 双喜 the doubled-happiness character 囍 is pasted at every wedding. Singles' Day (11.11) was named 光棍节 precisely for its four lonely 1s — the anti-双.
Two hands 又又 raised together — a PAIR.
双十一 'Double Eleven' turned the loneliest date into Earth's biggest shopping day — hundreds of billions of yuan in 24 hours.