Are the events in this manga panel happening simultaneously?
This would be a personal answer based on my own experiences: in cases like these I interpret the height of the speech bubble as order. The left bubble is higher up than the right one, so that happens first. There's nothing stopping the right one to be placed higher up, as it wouldn't hide any important details or anything in the art. Therefore I would read the bubbles in this panel left to right. 

The following image is from http://shadow.randomfandom.com/gh/styles.htm and shows reading order. Look at number 4 and 5, that's how I interpret the two speech bubbles in your example.