Google's John Mueller confirmed on Twitter that having space separated attributes in the nofollow attribute will still work as a nofollow.
He said rel='external nofollow' is the same as rel='nofollow' for the purpose of ensuring Google doesn't follow those links.
Here is John's tweet:
He then confirmed it once more:
This is nothing new to most developers and SEOs, but just in case anyone was wondering...
Forum discussion at Twitter.