Since danne's suggestion to use 2 card readers, I've done several videos using card spanning with mlvfs for instant grading/editing in resolve, with absolutely no issues!
the latest example:
https://youtu.be/o9w99l9zFgc
workflow: stick cards into readers, create 32MB ntfs ramdisk, run "mlvlinks" script, run mlvfs script, jump into resolve!
see this thread for details:
https://www.magiclantern.fm/forum/index.php?topic=25473.msg243387;topicseen#msg243387