Usually,
sweep rowing shells are rigged in such a way that rowers alternate sides. "Tandem rigging" is when two consecutive rowers are on the same side, made possible by removing and replacing the selected
riggers. Obviously, tandem rigging can only occur with fours and eights.
Tandem rigging is relatively rare, and is usually introduced in special situations. It takes oarsmen a while to get used to any sort of tandem rigging. The oars of the lucky folks in the tandem seats are in close proximity to each other, which tends to be very awkward. In addition, depending on how the rigging is changed, strange things can happen, such as the bowman and stroke being on the same side. This isn't necessarily a bad thing, but it takes a little getting used to.
There are advantages to be had from tandem rigging, however. Though it would be a coach's dream if his rowers were all clones, this is never the case. Each member of a crew has different abilities with regard to power, height, weight, and flexibility. Strokes typically are flexible and have excellent technique and rhythm, while the rowers in the middle of the boat are heavy and powerful (hence the term engine room). Rowers in the bow have to have a good sense of balance, but are not as powerful. Since each member of the team varies, lineups have to be selected very carefully. There are a million factors to be considered when making lineups. Certain people will follow other people better than others, and the way the weight is distributed in the boat has a very profound effect on speed. Tandem rigging allows lineups to change without having people switching sides.
There is only one type of tandem rigging possible in a four. The bowman and stroke are on the same side, and two and three are on the same side. In an eight, any seats can be put in a tandem rig except the bow pair. If the bow pair were put into a tandem, the boat would row in circles. I also don't think you'll see too many boats with their stern pair in tandem. The most common kind of tandem rigging in an eight is the "bucket rig" -- for example, starboards in seats stroke, 5, 3, and bow, and ports filling in the other seats.