JMich: I assume the same as always. If you have 1 license that comes on different platforms (aka drm-free, steam and/or desura), it's one license. You either keep it all or nothing.
If it's more than one license (aka 2 steam keys), feel free to do with the second what you wish.
Arteveld: I was thinking about it in a way i've seen with humble bundles, where people keep their drm free copies and give away steam keys.
No. Please don't. In humble bundle you have 1 license, and if you keep the drm-free copy yourself while giving the steam key to someone else, you are creating a second license out of thin air. While I don't have a problem giving the steam key if I already have it on steam, it is still a bad habit, and one that HB asks us not to do.
Xenonauts gives you 1 license, and allows you to choose which platform to use. But it's still one license.