We have 2 adults and 2 kids who are really good eaters (ages 10 and 12). We always just order two and share, because after we get the pommes frites, and still want to eat the begniets afterward...that's just plenty for us.
On our upcoming trip we have debated ordering two orders of pommes frites and just one sandwich to split between the four of us. I actually think that would be fine.