I'm going to vote for the reverso duo, despite the jibes - wore it to my wedding 😁
Not mentioned previously but could also work -IWC Portugeuse or Portofino or perhaps one of a few different Nomos models haven't been mentioned yet, Baume & Mercier perhaps, Cartier
No chronos! 🤦- think simple there-hander time only !!! - its a formal event if they have specified a tuxedo, do the happy couple to be the courtesy of observing their wishes.
As others have mentioned, metal to match your other accoutrements - shirts studs, cufflinks to match watch metal or colour.
Oh, and it goes without saying, no divers either!