Not practical as such, because of the devaluation if you "launder" the points through a mutual partner of UA and DL that permits transfers both ways.

What you can do, depending on what you want the DL miles for, is find a friend who has DL miles and is willing to give you an award from them. You might then later give your friend an award using your UA miles.

Doing this as a barter transaction violates the terms of every FF program I know of, including these two. Mutual gifting among friends on a small scale is unlikely to arouse suspicion. A public proclamation of interest in doing this, via Craig's List or anywhere else, stands a good chance of catching UA's attention. At that point you're likely to have your account closed and lose all your miles. (Airlines have staff to figure out who posts ads for this. This is their job. They do it well. The sham of "I'll sell you a plain white envelope, there will be a gift of a ticket inside it" is worthless, in law and in practice.) So, if you decide to try this approach, be careful how you go about it.