If your doing overhead DB tricep extensions with two hands than your doing a French press, which can be better served when using a EZ bar for a lot of reasons. One arm DB extensions are a great exercise for mass and also insuring that both triceps get equal development in size and strength. Gives better balance to both arms so one does not appear larger than the other (though by nature one arm may be a slight larger/stronger that the other). It may or may not be a good tricep exercise but it's rare to see any one doing tricep kick-backs for muscle mass.
There is the strict version where the bicep is pretty much held/pressed against the side of the head and the DB lowered close to the back of the head towards the opposite side of the shoulder. Takes more flexability, with more focus & stretch on the triceps. Some will have a training partner press and hold the working arm against the side of the head each rep. Or lean against a wall, piece of equipment,etc to keep the exercised arm tight against the side of the head.
The other version is to lower the DB straight behind you. Some guy's will have the elbows pretty much pointing up in the air while others have the elbows a little more than a 90 degree angle, touching the delts every rep. Any version can use cable's
Seen some guy's who work with a training partner, use a strap (judo belts are good...thick and strong) rather than a DB/BB. Having the partner behind him (usually sitting) and apply resistance (both ways..positive and negative...keeping the resistance on while fighting the negative pull back by the partner) while the BB'ers does full tricep extensions. Some of the more outstanding pumps I have ever seen are accomplished this way. If anyone have never experience doing positive/negative resistance in the same exercise, they are in for a big surprise. Though most of the time these strap/bely tricep extensions are done while laying face down on a mat or bench, rather than standing. If ever doing positive/negative tricep extensions (can work well with the biceps...trainee is lying down on a mat, bench,etc for best angle) have it as the only exercise for whatever muscle group being worked. Good Luck.