What is duplex mode ?

For some Blackmagic cards that support multiple inputs/outputs (DeckLink Quad 2 and DeckLink Duo 2), you can configure how connectors are related.

Connectors works in pair. They can be configured as half-duplex or full-duplex.
In half-duplex mode, either connector can be an input or an output.
In full-duplex mode, one connector is used for input, the second acts as a loop output of the input.

How Multicam deals with duplex mode

By default, Multicam automatically set cards to half-duplex mode. So you can use each connectors independently.

If in your setup, you need to use your cards in full-duplex because you want to loop signals, you will have to edit configuration file.

How to tell Multicam to not set half duplex mode

You have to edit Xml configuration file located here:

C:\ProgramData\MultiCam\Recording\software.multicam

Look for SetDecklinkDuplexMode node in the file.

By default, you will find this:

<SetDecklinkDuplexMode>true</SetDecklinkDuplexMode>

Change it to :

<SetDecklinkDuplexMode>false</SetDecklinkDuplexMode>

Now, Multicam will not change the configuration you made in Desktop Video Setup tool.

Configuring duplex mode

Configuration is made using Blackmagic Desktop Video Setup tool.
For a card supporting duplex, you will find a Connectors section:

If you choose SDI 1 in the drop-down, the card will be set to half-duplex.
If you choose SDI 1 & SDI 2, then you will link the two connectors, and the card will work in full-duplex mode. So SDI 2 connector will output the signal captured via SDI 1 connector.