AS BR notes, there are a plethora of previous threads that address your very question.
But, the bottom line is... a regular rear bike mount that is designed to carry two regular bicycles is more than adequate for a tandem; do the math: a tandem usually weighs about twice as much as a regular bicycle of similar quality / configuration, e.g., 2-18 lb road bikes = 1-36 lb road tandem
However, I'm one of the ones who will always recommend removing both wheels when you carry a tandem on a rear mount -- which is really an easy thing to do since a rear mount hold a bike up off the ground just like a work stand -- so that:
1. the weight of the tandem is more centered
2. the tandem is more stable without the wind buffetting against your wheels
3. without the wheels a tandem's rear drop-outs and fork don't stick out much past the mirrors, noting that in some states it's illegal to have a 'load' that's wider than your car, and
4. you can actually see what's behind you with your side mirrors since the wheels don't obscure rear vision
Here's another thread that has some links to photos of cars carrying tandems on rear mounts. Again, my favorites are the last two which depict a Cannondale tandem on the back of a Porsche Boxster.
Too Short to Tandem on Trek T900?