If you connect two buttsets in series with a 48V battery you can talk if both buttsets are in talk. If ether buttset is in monitor the other buttset cannot generate voice. With both buttsets left in talk the battery will run down. If you connect two buttsets in parallel with one battery at one end the buttset on the same end as the battery will have all of the line current and the buttset on the far end cannot be herd. You will need an inductor in series with the battery to keep the voice signal from shunting out thru the battery. If you have two batteries with inductors one on each end then each buttset can be heard. Leave the buttsets in monitor when not talking and the batteries will last longer. This circuit has no signaling. The Viking DLE-200B Ringdown circuit provides signaling but spec says it will only work up to 2.6 miles on 24AWG. This would be about 10 miles on 19AWG. There you have it.