Yes, they can be used together, although the units are mutually exclusive (they decrease each other's morale severely and tend to debuff eachother if they are allied).
The thing is that the original mod Army of the Light doesn't include an english translation, and Army of the Dark's translation is a little poor (but understandable)
Another problem is that they don't work properly with the english version due to missing pictures, duplicate file hashes, improper translations, etc. The missing pictures can lead to crashes though. This doesn't happen with the russian version at all. The mod works perfectly with it.
__________________
|