You can redeem DL miles for a UA flight, just call DL to redeem the award and tell them you want it on UA. You can check UA availability on their website, then when you find the flights you want there, just write it down and call DL (You will need a UA FF account and need to sign up for online access to do this). This is kind of important, because I've had a situation in the past where DL will do a search of UA flights, and it won't show a flight that I can see on the UA site, but if they specifically type that flight into their computer it will show up. You can only redeem for UA domestic tickets however, not international.
You can also earn DL miles when you fly UA, just give them your DL SkyMiles number when you check in for the UA flight.
Finally, I think this arrangement might be ending in the near future ... not sure, but with all award travel it's always a good idea to book as far in advance as possible.