The Marriage of Carole Lombard and Clark Gable
Carole Lombard and Clark Gable were apparently both extras in Ben-Hur: A Tale of the Christ (1925) and The Johnstown Flood (1926). It's quite impossible to tell whether they spoke, acknowledged each other, or were even present in the filming—I wouldn't personally count this as the start in their timeline, but it's worth being noted that it's a possibility.