"With the Hirebrands and two students, Staff-Elder Asuraka learned that messages may be spoken and heard through lomillialor, the High Wood of the lillianrill. The task is difficult, and requires strength - but it will not surpass any Lord accustomed to the Earthpower."
I think this more or less states that if you are using the
lomillialor you can send
and receive messages.
Troy was afraid that the Lord had damaged himself. But Mhoram only suffered from a sudden exhaustion - the price of his exertion. All his movements were weak, unsteady, and his face dripped with sweat, but he managed a faint smile for Troy. "I would not care to be Callendrill's foe," he said wanly. " He is strong, he sends riders to Amorine." ~ The Illearth War, Doom's Retreat
We know that Callindrill does not have
lomillialor, so this verifies that this is a single unit form of communication. As long as one of them has the rod, they can speak, and this one sided mode is draining on the sender. The question remains, can you use rods at both ends?
"Mhoram," he murmured through the pauses in his song. "Mhoram, Son of Variol and Tamarantha. Open your heart to hear me." ~ the Unfettered One, The Power that Preserves, Message to Revelstone
I think we can assume that the Unfettered One did not think that Mhoram carried the rod around in his pocket, and he seems to believe he can succeed in the sending, even though we have no proof that he could have succeeded. This somewhat verifies that only one rod is necessary. It also reinforces that it does not require a Lord to communicate this way. I think this means that the Lords' melding has nothing to do with this. After all, Lords did not invent the technique. And it also show that varying forms of channeling Earthpower can be used.
In her (Amatin) hands she held the lomillialor communication rod which the Loresraat had given to Revelstone seven years ago.
later:
Mhoram understood instantly that she was receiving a message from whomever it was that held the other communication rod, the one at Revelwood. ~ The Power that Preserves, Variol-son
This says a bunch! It is possible to communicate rod-to-rod, and when doing so it is not necessary for Amatin to use her staff, as Mhoram was required at Kevin's Watch. Also, this form of communication is exhausting, but not so much as the single rod mode.
So my conclusion is:
1. It is possible to send and receive communication with only a single rod.
2. Communication is easier when using rods at either end of the message.
3. Using communication rods is draining, but less so in the two rod mode.
4. It does not require a Lord to use the rods, just a devotee to Earthpower.