Planning as a couple often means "who has the latest spreadsheet?" Crossed messages, duplicate tasks, RSVPs updated by one partner only.
Fiancé assumes each partner has a phone. Sync is optional: you can also pass one device on Sunday night if you prefer.
Typical split (adapt yours)
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Partner A: vendors, budget, timeline
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Partner B: guests, RSVPs, communications
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Shared decisions: seating, menu, theme
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Weekly 30-minute check-in: checklist + next deadlines