The version of vMix that has a multicorder is $630 (you have to get the 4K version).
Multicording is a very demanding process, expensive both in price and system resources. The Studio version of OBS may have it one day, but it will not be for long time. To my knowledge, there is no program that does this for free, because to be frank, offering this feature for free is kind of insane. It will be free in OBS if it gets added, but like I said, that is a long way away.
If you want to do fancy things with video production, it's eventually going to cost you money somewhere. OBS lets people get by with a lot of stuff for free, but there is a limit, and you have hit the current limit.