“…quod et caput et pes ei dolet.”
I felt certain that this would have been discussed here before, so looked it up.
Found this:
"…When the different subjects are felt together as constituting a whole, the singular is used; as,—
temeritās ignōrātiōque vitiōsa est, rashness and ignorance are bad."
Is this the relevant explanation?
I’ve noticed the use of “est” in lots of sentences in this way (in llpsi).
"Subject-Verb Agreement
In Cap. I, you learned that a singular subject is joined with a singular verb and in Cap. V, that plural subjects are joined with a plural verb. In this chapter, we see an exception to that rule: the verb is in the singular if there are two subjects separated by neque…neque, aut…aut, or et…et, as in:
et caput et pēs eī dolet. (ll.3–4)
nec caput nec pēs dolet. (l.66)
The general rule is that two or more subjects:
• take a verb in the plural if they denote persons, as in: Parentēs ā fīliō
intrante salūtantur. (l.91)
• if the subjects are things, the verb agrees with the nearest subject,
as in:
pēs et caput eī dolet. (ll.3–4, 64)"
From Lingua Latina A Companion to Familia Romana Based on Hans Ørberg’s Latine Disco, with Vocabulary and Grammar by Jeanne Neumann. p. 121
Edit you can find a more comprehensive explanation here https://dcc.dickinson.edu/grammar/latin/verbs-and-subjects