There is zero chance of two Nationalists winning. Last time round Alban Maginness was more than 18,000 votes adrift of Diane Dodds, and taking the undistributed surpluses into account the real difference was probably 24,000 (Nicholson ended with a 11,000 surplus, de Brún with 5,000) and the 7,500 non-transferables from Allister are probably a high-water mark for hardline plumpers. The total Nationalist vote is consistently 5-10% behind the total Unionist vote, and Unionists are better at internal transfers; there is absolutely no reason to expect 2014 to be any different.
There is also zero chance of Nationalist transfers deciding which Unionist candidates get elected. This would require the total Nationalist vote (impossibly) to fall below 37.5%. At every election since SF came into the system, the trailing Nationalist party has survived to the last count without being eliminated.
Even in the (impossible) Nationalist meltdown scenario, while it is wrong to say that no Shinner will ever vote DUP or vice versa, the numbers that do are likely to be too few to make the difference. The one occasion when something like this happened at a European election was in 1979, when Bernadette McAliskey was eliminated, and 81% of her votes did not transfer, failing to help Oliver Napier close the gap with Jim Kilfedder (let alone with Harry West or John Taylor).
If I were the DUP I would run two candidates in the hope of squeezing out the UUP. Jim Nicholson will be 69 by the time of the 2014 election and will have served five terms, so I would not count on his running again, and it’s difficult to see who the replacement would be (but then again few of us predicted either Allister or Dodds as a DUP candidate for Europe). The DUP have comfortably outpolled the UUP in every election in the last ten years except the last Euro-election. In both Assembly elections they got more than twice the UUP total.
But I am not the DUP, and I think they will run only one candidate. The fact that the one election where the UUP finished ahead of them (after transfers) was the last European election will certainly play into their preparations for the next one. Also the DUP have shown a commendable caution about over-nominating in recent years, even where their candidate might have had a good chance on the numbers. My impression is that internal discipline is strong enough that running two candidates would not be a big issue, but the party’s political aims are served more effectively by winning one seat safely than by scraping (or worse possibly failing) to win a second.