If two people arrive together but leave on different days, their onward reservations should reflect those different departure plans. There is no benefit in squeezing both travelers onto the same return flight simply because they share an inbound booking, a hotel room, or a visa appointment. A clear itinerary for each passenger is easier to understand and more useful than one tidy-looking document that gets the dates wrong.
One arrival does not mean one return booking
Picture two travelers flying from London to Kuala Lumpur on November 4. Maya plans to return to London on November 13. Daniel stays until November 21 before flying to Singapore for a work meeting. If both names appear on a November 13 return reservation, that does not accurately describe Daniel's trip. If both names appear on November 21, it misrepresents Maya's plans.