See this thread for more details: Since When is DL charging per segment on partner awards?

What is likely is that there is no published revenue fare using these two partners between the two cities. Essentially, the DL system is broken for circumstances like these (arguable if this is intentional or not).

What you need to do is call and ask an agent to price this itinerary, if it comes up with double the normal price, ask them to price it segment by segment manually. If they resist, ask for a supervisor or HUCA.

You may never get it to work, I have failed more than I have succeeded. But that should be the best way to try. Because DL does not publish any award pricing, you really can't argue with any documentation against whatever the system prices, but you can give it a shot.

Another reason that Skymiles is one of the worst FF programs out there. Good luck.