When two singular noun phrases connected by either/or or neither/nor together function as the subject, they normally take singular verb agreement in formal English:
(1) [Neither Jan nor Anna] is (are) available.
(2) [Either James or Carla] is (are) helping today with the presentations.
The two singular noun phrases in each example are highlighted (boldface), and so is the singular verb. The corresponding plural verb appears within parentheses, to indicate that this is an alternative in less formal types of writing and speaking.