I have a friend who had kids in the UK, then she flew with her LO and her older son (who was 2 at the time) to the USA ... but her husband had to join her as the airlines wouldn't let her fly alone with two-under-two : that's a rule of some sort.
Also, last time I was on a long-haul flight and a couple sitting together were holding their 2 LO's in their laps. The flight attendants separated them (first one told them the rule, then the head attendant was doing a final round of the plane and he told them the same thing too) that you can't have two kids with no seats in the same row. So apparently you have to sit in separate rows if both of you are carrying kids so small they didn't need their own seat.
Just letting you know in case you have +1 kids