Who would be at fault?

I almost witnessed this accident a couple days ago, but both vehicles realized and pulled back into their lane. They were parallel with eachother and both wanted to change into the lane between them. Presumably they didn't see that the other was also looking to go there because the signals are hard to see from the side, and only really checked if someone was in the lane immediately next to them. They both indicated correctly.

I hadn't considered this kind of accident before. Who would be at fault?