Pretty much as I remember 2 ACD groups 1 for each card, the main dialin (Main DN in NTP) number is an NACD that has the 2 ACDs in it.
Like Nortel4Matt says the existing card should be set as the Dual card Primary (in the setup wizard) and the new one as the secondary. Then there is another couple of numbers for the TUI (1 for each card), Chair Person (only Primary card), Transfer and Link DN's set up on Primary card pointing to the ACD of the Secondary card. Most of the setup is done on the primary card.
The cards can be programmed with a single DN for dial in or Pairs of DN's (different numbers for different Conferences (I Preffer just the 1)). The NTP is quite good. Conferences can be defined on either card but Dual conferences are only defined on the Primary card.
In Dual Card conferences there is an option for full control of the secondary card if this is selected 2 ports are connected between the cards so you lose 4 of your channels (2 on each card), with Full control not selected you still lose 2 Channels (1 on each card) for a speech path between the cards.
Also the admin user can only set up static Conferences and a Dual card Conference cannot be static so has to be set up by a normal ICB User.
Thats about all I can think of.