Antiphonically

adv. [f. prec. + -LY².] In an antiphonic or antiphonal manner; antiphonally.

1846.  Maskell, Mon. Rit., I. Introd. 34. Portions of the services … sung … antiphonically.

1851.  Q. Rev., No. 177. 237. The singers, for their own ease, sang them antiphonically.