Sorry, but there is no way to distribute a double-disc album so that it appears as one (1) release in the digital stores.
You have two options if you want to release a double album in the digital stores:
1) You can distribute two (2) separate albums and title them "Vol. 1" and "Vol.2." With this option, you will receive more money from your sales but you would have to pay to distribute two separate releases.
2) You can distribute all of the songs on both discs as one (1) album and charge a maximum price of $15.99, in iTunes.