Flower sharing

I have watched many nectar sippers on flowers. The arrival of a second one usually drives the first away.

Today, I was watching both a Drone Fly (Eristalis tenax) and a Western White Butterfly (Pontia occidentalis) roam around a field of Asters (Canadanthus modestus?) as they sipped nectar. What was unexpected was that at one point they shared the same flower for a while with no apparent sense of conflict.