HME DX121 Wireless Com

I have used the DX210 system, but have not seen the DX121.
The specs say that the DX121 can register up to 4 beltpacks, but only 1 can be used full-duplex. The other 3 are half-duplex.
What is not clear is if that is a limitation only between the beltpacks and the wired interface (IC mode), or if the beltpacks can still talk to each other full-duplex in ISO mode.

Anyone have any experience with this system?