Directshow output with multiple cards

I have 2 Decklink SDI 4K cars and I'm trying to use both of them to output video at the same time. For some reason, I can't make Blackmagic WDM Render filters work, which leaves me only with the deprecated Decklink Video Render, which would be fine if it would allow me to select which output card to use.

Is there a way to use 2 cards for output at the same time? Or is there anything that has to be done to make the WDM Renders work? WDM capture filters work fine.

Adding some more info, when I try to connect directly a WDM capture filter to a WDM render filter, I get a 0x80070020 The process cannot access the file because it is being used by another process.

On a side note, I'm not getting a "Decklink Video Renderer" for each device connected, only one is active. Is there a way to fix that?