well with a central unit like that, space isn't so much of a constraint, so you could have a 2.5" laptop hard drive (ruggedized, better able to handle vibrations) so you could easily have a terabyte of fast storage for all channels to write to simultaneously.Whilst I am all in favour of a single DVR recording from 2 or 4 remote cams, I would prefer that the 2ch (for example) be (something like) 2 Mobius-sized cams, each with its own lens on the end of a long lead.
Have a single power supply feed the box & the electrickery inside splits the power to feed each cam. More electrickery inside the box monitors the state of each cam (have they both started recording, sync the time etc...). I would also hope that this central box would have a screen so that you could review footage from each cam. If desired, relevant files from each cam could then be transferred to an SD card in the form of cam1, cam2 files.
I honestly believe that it is just too much to expect hd video from 2 cams to be recorded simultaneously on the one SD card
so you could have a 2.5" laptop hard drive (ruggedized, better able to handle vibrations) .
True, but also much more expensive per gb. A laptop hdd with rubber or silicone isolating bumpers should do just fine in all but the most severe rough roads or stiff suspension cars. I had a desktop 3.5" HDD in my car for a year and a half as part of a pc for playing mp3s back in the early 2000s. It was hanging by springs in the center console with foam on the walls in case it bottomed out. No issues the entire time.SSD, fast and no moving parts