That would be in fact a cheaper solution.
But i can still get it cheaper
I think that it would be cheaper, price wise, to go with 2 DeckLink Mini Recorder insted of the Decklink Duo and you would still have the option between HDMI and SDI that way.
Also, instead of goig for vMix, just use Open Broadcaster Software (OBS) which is free and that way you just have the cost of the hardware.
I belive it's going to be hard to find any cheaper options, but the challenge is hereby launched
Best regards,
Paulo Brinca
Don't settle to be good, make shure you can be great...