That's not true at all.
Yes, you can throw them both into sound active mode, and they'll both behave in sound active mode, but they won't be doing the exact same thing at the exact same time. When you go "sound active" in DMX, they both respond to their own internal mics.
Don't confuse DMX Sound active mode with stand-along (with master/slave) sound active mode, in which case they WILL do the same thing because they are configured as a master and a slave and then chained together.
Also, don't forget, don't mix "master/slave" modes and DMX on the same cable run. It won't work.