مَرَرْتُ بِزَيْدٍ وَعَمْرٍو
Translation: I passed by Zayd and ʿAmr.
Bi-Zaydin: preposition plus genitive noun; ʿAmrin: genitive coordinate of Zayd.
Here the first conjunct is an overt noun. The text will state an additional condition when the first conjunct is an attached genitive pronoun.