A two handed rod is better than a single hand rod in a number of ways if you have the room.
Two handed fly rods have been used in Europe for a long time and are gaining popularity in the United States. They range in length from 10ft to about 20ft. In the States you'll find most are 10 to 15ft. In comparison, most single hand rods range from 7 to 10ft. Two handers are used in fresh and salt water where one may need to throw a long line to reach the fish. With an overhead cast you can really generate a lot of line speed needed for distance. Another place you will see people using them is on the salmon and trout rivers along with spey casting techniques. Here again the length is advantageous. You can cast farther and control the line better on the water. You'll also see these rods in casting tournaments where maximum distance rules. I'm sure we'll see more uses as their popularity grows.

Other advantages and disadvantages of two handers. Using two hands on the rod when casting is easier on the body. I found that after casting for salmon and steelhead all day that I have less fatigue and joint pain compared to using a single hand rod. Long rods are not practical on small water. An 11ft switch rod works well for the size river in the picture above. If there is a lot of brush on the banks you'll have to break the rod down to get through. Landing a fish by yourself can be tricky. You have to reach the rod way out to get the fish close. You could have someone else land it.

How to go about selecting a rod. Most major fly rod manufacturers now sell two handers. Common lengths are 12-15ft and commonly called spey rods. Rods 10-11ft long are called switch rods. They are designed to be cast with one or two hands, thus the name. They are ideal for smaller rivers where long spey rods are not practical. Among the two handers you will find rod actions vary widely. Slow actions will cast more gentle open loops like those used in traditional spey casting and be more forgiving of casting errors. Others are fast action and will throw tight loops into the wind. This is the action to use for tight looped modern spey casting and for over head casting. You'll find spey rods are designed to cast spey lines, skagit lines, and scandi lines(I'll cover lines in the future). Switch rods are designed to cast skagit lines, scandi lines or standard fly lines. The spey, skagit, and scandi lines are a lot heavier than standard fly lines so find out what the rod is designed for. With overhead casting a variety of lines will work well, but for spey style casting you'll find the line is more critical. I tried four different lines that I had and found one that worked well with the switch rod I just built. I have heard of others trying many lines before they found the right combination. Your best and cheapest option is to find someone that knows what rod and line combination work for your fishing style and situation.

Two handed rods and spey casting techniques are fairly new to the United States. Fisherman and manufacturers don't have it all figured out yet. Techniques and equipment are in constant flux. Some of the mystery will play out over time.