I tried without success in September of last year to book a single DL ticket using UA miles for travel in July of this year (so I could save some of my DL miles); I was, however, able to reserve a total of 7 tickets for the same travel dates on DL using DL miles. The year before (2001) I tried to no avail to book award travel in June 2002 to HI on UA using DL miles (UA served more destinations within HI), again many months in advance, but had no trouble booking 4 tickets on DL using DL miles for the same travel dates which were unavailable through UA. It seems to me that the seat allocations are very low for this type of cross-airline award, and that you are better off building miles on the airline on which you plan to travel using an award.