ad. In an antiphonal manner; in the manner of an antiphon.
The Century Dictionary and Cyclopedia
In an antiphonal manner; responsively.
Word Usage
"Already, half an hour ago Alcibiades and Yaxis had departed with their pushcarts, one to the north and one to the south, calling antiphonally as they went, in clear, high voices that came fainter and fainter to"