Let's assume there are two characters that have the same level, class, charisma score etc. Basically, they are identical when we're just looking at the numbers and stats. Both of them cast Command Undead on the same (unintelligent) target, and give two counter-active commands. Something along the lines of 'Look left' vs 'look right'. What would happen? Would the Undead first do one, then the other? Would it do neither? And if the Undead is intelligent? Would it follow the command it would be most likely to follow if it was not enchanted? The two casters are no allies of each other. In fact, they would probably be actively trying to wrest controll of the undead from the other.
Probably the competitors make opposed Charisma ability checks, and the creature obeys the competitor with the higher result
The Player's Handbook on Multiple Mental Control Effects says
If a creature is under the mental control of two or more creatures, it tends to obey each to the best of its ability, and to the extent of the control each effect allows. If the controlled creature receives conflicting orders simultaneously, the competing controllers must make opposed Charisma checks to determine which one the creature obeys. (172)
There's an argument that can be made that an undead creature affected by two or more command undead spells isn't technically under mental control but magical control, but, absent other rules, this should suffice.