When I was a kid we would regularly ride two to a bike by having the passenger either sit on the cross bar, on the seat while the rider stood up in the pedals, as in
Double Drunken Bike Riding - or on a luggage rack. I still occasionally see people sitting on the handlebars though I can't imagine how they do it.
Most rear racks have weigh ratings. My Blackburn which attaches to the frame eyelets is only rated for 50 pounds and I believe Tubus racks are rated to carry up 100 lbs. Those seat post mounted racks have a maximum load of about 25 lbs.
Here in New York City it's illegal to carry more people than the bike was designed to carry but, if you add some fold up BMX pegs, maybe a nice pillow to fit the rack, that might be a moot point. I can't remember ever seeing anyone ticketed for doubling.