On a personal level I would not do that. Unless I made it very clear that whatever I was producing for the swap was from an edition and not a one of a kind piece.
For me participating in 2othings is about receiving hopefully a work that is unique and not part of a mass production.
I'd love to hear other opinions on this it's an interesting point to talk about.

I don't think that it's absolutely necessary that the 20 Things work be so wholly unique from the other work you're doing as an artist that you wouldn't have other similar work in other venues. I believe that artists who have participated previously have gone on to, for example, print more of an edition originally created for 20 Things. To me, that's perfectly fine, though my preference is that the work is created specifically *for* 20 Things and perhaps then extended rather than, say, sending in prints 47-67 of an existing edition of 250.