"Either/or" and "neither/nor" can be singular or plural.
When "or" or "nor" joins two elements, use a singular verb if both elements
are singular. However, if one of the elements is plural, use a plural verb.
In these examples, the elements are shaded.
Shortbread or cake is on offer. correct tick
("Shortbread" and "cake" are singular.)
Chocolates or cake are on offer. correct tick
("Chocolates" is plural.)
Be mindful though that many writers follow the Proximity Rule. This states
that the element nearest the verb governs it. For example:
Chocolates or cake is on offer. correct tick
(Using the Proximity Rule, the verb becomes singular because "cake," which is
the nearest element to the verb, is singular.)
https://www.grammar-monster.com/lessons/either_or_plural_or_singular_verb.htm
Are either or both of the following, redox reactions?
Source: Chemistry With Inorganic Qualitative Analysis, 2nd Edition, p487.
DOS keeps track of the current default drive, and any time you or a program
refer to a disk without giving a drive specification, DOS assumes you intend
to use the default drive.
Source: Peter Norton’s DOS Guide, page31.