When singular subjects are connected with neither...nor, the verb is usually and more acceptably singular,
"Neither cat nor dog is allowed."
[This] is because neither...nor is a negative counterpart to either...or, which is singular.
But neither ... nor is also a negative counterpart to both...and, which is plural,
"Neither cat nor dog are allowed."
<Meaning, Both cat and dog are not allowed>
But if both subjects are plural, the verb is plural:
"Neither cats nor dogs are allowed."
If there is a singular and a plural subject, it is most acceptable to put the plural subject second and have the verb agree with the nearest (plural) subject,
"Neither the cat nor the dogs are allowed."
Even if the plural subject is placed first, it is common to use a plural verb,
"Neither the cats nor the dog are allowed."
Read more ...