The basic premise of Dual Hidden Type is that it is a variation of Hidden Type, but instead of one type, users are able to select two types. Here's how that works.
Phase 1
Players PM the host with their types. Typically, it will be in the order of Attacking / Defending.
Here's a visual example to put it into perspective:
Player 1: Dark / Water, attack Player 2
Player 2: Fire / Psychic, attack Player 1
Host: Player 1 attacks Player 2, the attack is Super Effective
Host: .roll 125
Host: .roll 100
Host: Player 2 attacks Player 1, the attack is Not Very Effective
Host: .roll 75
Host: .roll 100
The attacks played out in that way because Dark is super effective against Psychic, while Fire is not very effective against Water. If it were Fire vs. Normal instead, then the roll would be 100 for both.
Phase 2
Attacks are carried out similar to the example above.
See Hidden Type page for details on what attacking/defending rolls should be depending on the effectiveness of attacking types on defending types.
AI is on.
Types do not change in-between round unless Protean mode is turned on.
Modification by Lunar