El Capitan Beta 2 upgrade fails

If your system didn't boot into SUM then that would certainly throw the hardware into question. I haven't read anything yet though that indicates to me that it likely is a hardware problem. You could boot holding D to initiate the Apple Hardware Diagnostics (https://support.apple.com/en-ie/HT202731) just to be sure.

There was, however, a fimware update included (seperately) with DP2. Possibly that could have been the cause. You could test it easily enough by only updating to DP2 and the Recovery HD update (the other two components of the recent update). That should be easy enough to do from within the App Store - if not, the individual packages are available directly from Apple's CDN. https://forums.developer.apple.com/thread/6606

Diagnostics are clear, no issues found.

After following steps:

Partition

Install El Capitan Beta1 on the internal PCIe drive

Reboot configure, everything is fine.

Install Recovery from the pkg.

Install Beta 2 from pkg.

It all installs fine.

After reboot the system is hanging after SUM

That sure is peculiar. Perhaps a kernel crash due to some bad coding related to your particular config. Good you opened a bug report.

I am thinking along the same path.

There is nothing special on the Mac Pro (Late 2013 ) , Six Core , 1 TB 32 G and the Dell Display.

And assuming that you opted to "Update All" (i.e. all three packages) from within the App Store in the first instance, that would rule out the possibility that the Firmware Update actually addressed this issue.

Yes.

The installation was fine.