This will result in half the intended impedance being reflected back to the OT primary; may not be an issue unless it's got EL34s and a high VB+.
Is there a 4 ohm output? If so plug the 8 ohm speaker into the 4 ohm output and the 16 ohm speaker into the 8 ohm output and the correct impedance will be reflected back to the primary.
Pete.