Hello,
Recently I began using multicoder/recorder in vMix. But unfortunately my PC can't handle recording 2 separate capture devices and also sending a stream. I've got a spare computer with same specs and wonder what is the best way to use 2 instances of vMix at once. How could I send video/audio stream with minimal effect on hardware. NDI? HDMI?

Try NDI first, given that it doesn't cost any extra - assuming you can link your two PCs via a gigabit LAN. Set the capture inputs to be converted to NDI feeds by vMix and then record them on the 2nd PC, either by vMix or by using the built-in Record function in the NDI4 version of NewTek's Studio Monitor.
On the other hand, if your main vMix PC can't handle the extra load of encoding the capture inputs to NDI, then I'd suggest splitting the input signals externally (via active splitter boxes) and performing a separate capture on the second PC. Of course that would involve addition capture devices, which is why I'd try the NDI option first.