Got my StereoPI v2 about a month ago.
I have always got difficulties on booting the SPI in the way where the both of the cameras will boot up. Usually about every twentieth time the SPI will boot with the both of the cameras. Most of the times there will be no cameras visible and ocassionally only one camera will boot and then I can see only one preview image on the screen.
Restarting the SPI over and over again I will eventually be able to start the SPI with both of the cameras working correctly.
I have tried to use several different power supplies, one of them is original RPI USB-C power adapter. Also tried to use it with a mobile power bank. I even bought an RPI UPS module equipped with a three 18650 batteries to get enough voltage to run the SPI.
It doesn't matter how I power the module, the cameras won't show up until I keep restarting the SPI. I have checked the cables many times and even tried to use the long cables as well. Nothing helps.
Do You have any advice what to do with this?