In theory this should be possible.

A few things to note:

• vMix doesn't know/care how the cards are hooked up. As long as they're visible on a system bus they should work
  
• You may run into bandwidth limits depending on how you have your enclosures hooked up (daisy chained vs each having their own port)
  
• The Decklink Duo 2 card is a PCIe 2 x4 card. This means it'll take roughly 16Gbps in bandwidth.
  

I'd recommend checking Windows Device Manager to see if both Decklink cards show up. If not then work on debugging that. If you see a message about 'device cannot start' it's likely you don't have enough bandwidth to run both cards on your Thunderbolt setup. From what I see online, your laptop has a single Thunderbolt 3 port (which gives ~40Gbps bandwidth). With 2 enclosures daisy chained and each card using up 16Gbps you're likely running out of bandwidth. (It's possible each enclosure tries to reserve a full PCIe 3.0 bandwidth even though the card only needs 2. That right there would definitely eat up your bandwidth).

I realize you probably already own the enclosures but I'd recommend finding an enclosure with 2 PCIe slots as the reduced overhead of only having a single enclosure may give you what you need.