Tandems come in several forms. Racing tandems are still used in time trial competitions, although this is very much in the minority compared with its potential for more leisurely forms of riding. Tandems are not limited to just road use. Off road tandems with suspension are available from some manufacturers increasing the versatility of this form of riding. Touring tandems are also available, and this is one of the more popular ways to use them. They are strong and durable, as they must be built that way to cope with two persons body weight, so a bit of extra weight in the form of panniers and luggage is easily taken in their stride. Basically you can take a tandem pretty much anywhere you would take a solo bicycle.
Tandem frames are subjected to much greater loads, especially twisting forces as the riders body weight often moves in opposing directions as you ride. Because of this frame construction and materials need to be stiff and strong. Usually tandems are made of either steel or oversized aluminium tubes. Wheels and tyres need to be more durable too, as they must support much more weight, bearing in mind there are still only two wheels to support two riders. A greater number of spokes, especially in the rear wheel, with strong hubs and rims are a must. Tyres will benefit greatly from reinforced sidewalls, and some extra puncture protection is always a good thing on a tandem if you don’t want to have your enjoyment continually interrupted. Powerful brakes are a very important safety feature, to be capable of stopping a much heavier bike, particularly when descending as the extra weight means tandems pick up speed rapidly too. A parking brake is sometimes included, to stop the bike from rolling away from you when stationary on a slope.
Tandems are sized separately front and back, so that you can get a good fit for both riders, and ultimately a more enjoyable experience. A good tandem should also offer many of the same positional adjustments for both riders as a solo bike, e.g. handlebar reach and height, saddle height and fore/aft position. It is often beneficial for the stoker (rider at the rear) to have a suspension seatpost to soften the blow of bumps as they will not be able to see them and prepare themselves as they would when riding solo.