I was interested to find a meaning of "zeugma" that must have preceded the use of the word in rhetoric.

1. ZEUGMA and APAMIA (not to be confused with the Zeugma by Thapsacus where Alexander crossed
the river; cf. Strabo, XVI, 1, 23. Zeugma means simply ford, or crossing).

This is mentioned by Pliny, V, 21; "Zeugma, 72 miles from Samosata, a fine crossing of the Euphrates.
Seleucus Nicator joined it to Apamia on the opposite bank by a bridge."

How does its meaning of "crossing" come to be used as the rhetorical term?

zeug[ma 7zy1g4m!, z1g$38
n.
5L < Gr, lit., YOKE6
1 SYLLEPSIS
2 a figure of speech in which a single word, usually a verb or adjective, is syntactically related to two or more words, though having a different sense in relation to each (Ex.: The room was not light, but his fingers were)