There are also one hand DL's, as another method of boosting up strength for the two handed version. A very good exercise that falls into the class of forgotten exercises. Probably because their too hard to learn the balance and form for most people. Any one hand lift is ideal for an athlete, though not too many BB'ers are really athletic by nature. . After taking time with this exercise you could almost reach the same amount of lbs used on the regular DL. Another great way to build an outstanding grip. Usually a good idea to train each side of the body seperate (from time to time) from the other side anyway. Can insure both side will have close to equal strength and not one side stronger (dominate) over the other. This may also help with the problem of one side of the body being a little larger than the other; arms, thighs, pec's, etc. Both these exercises can be preformed with-in a power rack, also.
Two ways to do one arm DL's. Stand in front of the bar and use the center knuckling as a general gage. Though not all bars are balanced correctly, a little exploring will be require to find that true center of balance.The other way is to straddle the bar and lift that way. This is similar to the old Jefferson squat/lift where borth hands are used, one front and one back. Switch the hand positions between sets on this one.
There is also the hip/back lift, which is the king of all dead weight lifting. A heavy harness, and probably a special lifting platform, is require. Though the exercise could start from being lifted off the floor, the ROM would be limited. Huge weight can be lifted, which can have a positive affect on one's regular DL.
We use to do two man DL's. Lot's of fun to see how much two guy's could raise off the ground together. Not really a regular workout exercise, just did them for the fun of it. Good Luck.