Herbert Weir Smyth, A Greek Grammar for Colleges

Subject in the Dual, Verb in the Plural

[*] 955. A dual subject may take a plural verb: ““Ξενοφῶντι προσέτρεχον δύο νεα_νίσκω” two youths ran up to Xenophon” X. A. 4.3.10. In the orators the dual verb is almost always used.

[*] 956. The dual and plural verb may alternate: αἵρεσιν εἱλέτην τε καὶ διεπρά_ξαντο the two souls have made their choice and put it into effect P. Phae. 256c.

[*] 957. The neuter dual may be followed by the dual, the plural, or the singular verb (A 104, 200, M 466).