"Simultaneously" vs "concurrently"

I just looked these up in an online dictionary; this is what I found:

concurrently: overlapping in duration

simultaneously: at the same instant

Both of them mean "at the same time," and are almost interchangeable.

That said, if I had to point out a slight difference, I'd say that concurrently occurs over a longer time, and is a bit less synchronized than simultaneously.

For example:

The two processors were running concurrently, then they crashed simultaneously.

In that example, I wouldn't swap the two words in question.

In addition, concurrent has a few additional meanings, most of which amount to "in agreement," as in "two people working concurrently." That is to say, they're working towards the same goal. The other meaning of concurrent is related to geometry, where you might say two lines were concurrent: they're going to intersect at a common point.

EDIT:

[I don't have enough rep to comment beneath this yet, but I'd like to say I think you guys should stop commenting. I don't think you're adding anything anymore.]