I'm not aware of such a case. I highly doubt there is a usecase. Usually you have RAID in hardware but you want to do it yourself in software. Which is absolutely fine. But as you wrote, usually you have separate cases with separate power supplies and so on.
You can power everything with one power supply if you pick one that has enough power for all devices. 3.5" hard drive cases usually are powered with 12V so pick a stronger one which can power both HDDs the Pi with enough head room, get a splitter cable or build one yourself and use a DC-DC step down module like this or that.
The first one can power you Pi directly over GPIO, bypassing the losses of the microUSB plug. Only do this if you are confident with what you are doing. You bypass the fuse that way. Any connected USB device that draws to much current can cause damage to your Pi!.
In the setup with two self-powered devices however, this is pretty safe. But any alteration of the setup may be dangerous.
The second one is more fail-safe but you will have to also buy a barrel plug and solder a short cable.
