Ibi Orgetorigis filia atque unus e filiis captus est.
There the daughter and one of the sons of Orgetorix was captured. Why is the verb singular?
This is a bit of a minefield. Normally you are right it would be two subjects and a plural verb, But there are exceptions see Gildersleeve 285.
Kelsey says “When a Verb is used with more than one Subject, it may agree with the nearest Subject, or be Plural ; as, filia et unus e filils captus est, a daughter and one of the sons were taken captive (I. 26) ; Nammeius et Verucloetius . . . obtinebant, Nammeius and Verucloetius held (I. 7).”
Gildersleeve says that the “common predicate may agree with a sing. subject when that subject is the nearest or the most important.” Perhaps we are unsurprisingly to conclude that one of his sons was more important than his daughter.
I hope this doesnt seem too circular, but I think as Caesar wrote this it must be correct Latin, at least for his time. I suspect that this usage varied quite widely.
Latin verbs often agree with the person of the closest subject. In this case, the closest subject is ūnus ē fīliīs, and the person__ is singular, so the verb is also singular.
The two subjects, for instance, might be considered together in such a way that a singular verb is fitting. See DCC 317b.:
If the subjects are connected by disjunctives (§ 223.a), or if they are considered as a single whole, the verb is usually singular.
It then cites many examples, one of which is the following:
Senātus populusque Rōmānus intellegit> . (Fam. 5.8)
The Roman senate and people understand.
Alternatively, the two subject might be considered separately. See DCC 317c.:
When a verb belongs to two or more subjects separately, it often agrees with one and is understood with the others.
Intercēdit M. Antōnius Q. Cassius tribūnī plēbis. > (B. C. 1.2)
Mark Antony and Quintus Cassius, tribunes of the people, interpose.Hōc mihi et Peripatēticī et vetus Acadēmia concēdit. > (Acad. 2.113)
This both the Peripatetic philosophers and the Old Academy grant me.
The singular verb is especially fitting since the conjunction between the two subjects is atque, which has a more emphasizing rather than joining force than et. As Lewis and Short states in its entry for atque (IBb), atque can be used “in joining to the idea of a preceding word one more important, and indeed, and even, and especially”. Atque comes from at + que, so it kind of has the sense of “but and”, that is, “but also”, “yet even”. This would explain why the verb especially is drawn to the singular because Caesar is most likely putting more emphasis on the fact that one of Orgetorix’s sons was captured, rather than a daughter. In those days I assume it was more important for the general that he capture a son rather than a daughter. Caesar’s making it singular to make sure you know that he really did capture the son, rather than putting daugher and son together with et to lessen this fact.