To specify a specific card you can use the -D option and add a number at the end. This number will always be the same afaik as long as you dont remove/install cards.

For example:

DiabloMiner-Windows.exe -u user -p password -o pool.address.com -r port(e.g. 8337) -v 2 -D 2

Where 2 would be the number for your mining card. Since you have 2 cards, it could be either 1 or 2. Diablo tells you what card it's found and is going to use so trial and error will get you what number you need to put there.