X299 Extreme 4 No POST with multiple CPUs

I hear ya, love the attitude too. This kind of thing is best viewed as a mystery
to be solved rather than an annoyance. Being patient and methodical helps tremendously.
Good luck with the other build, I will be here when you get back to it.

No missing RAM beep means the board isn't detecting the CPU properly/at all. AFAIK
the board will not power on with no CPU installed but that safety just requires
a specific pin pulled low (like a jumper) in the socket so even a dead CPU
will allow the board to power up. So that doesn't help us at all. Make sure you
are installing the CPU the correct way around, the gold corner/arrow should align
with the corresponding arrow on the socket (top right of the socket if looking
at the board with the rear IO on the left hand side).

Make sure the CPUs are not warped/bent. The substrate (green PCB) is quite prone
to bending under pressure or even because of a drop. If the pads are not making
proper contact with the board then the CPU could fail to initialize.

The board's RGB lighting sadly tells us nothing as it is an independent system
on the board and doesn't provide any kind of code/troubleshooting insight. The
PC speaker is pretty much our only indicator here. A PCIe POST analyzer add in card
would be helpful but unless you build systems often it's a bit of a bad investment.
That would give us debug codes and some idea of where the POST process is failing.
At this point however it has to be installation issues. If you are not testing
with the board outside of a case already I would do so. That will rule out
shorts under the board caused by misplaced standoffs etc. Double check to make
sure none of the component legs have bent over and shorted under the board.

Throwing the troubleshooting book at it here, hopefully something sticks

That is odd, there should be something. Try without any RAM and see if you get the
no RAM beep. If you still get nothing then you may want to look at your CPU 12v
cables and make sure you are using the 4+4 cable not a 6+2. For some stupid reason
it's possible to force the PCIe 8pin into a CPU 8pin socket a lot easier than it
should be. It makes no sense that 2 boards could have issues or 2 CPUs are dead.

Good luck and post back once you have tried the above.

Great, without any debugging codes it's tough to gauge what's going on.

The i9-9820x requires BIOS 1.50 or newer to run on that board, could be that your
board has an earlier, non-compatible version. That would fit with the power cycling.

The 7900x on the other hand should be supported on all BIOS versions. If you have
a spare PC speaker laying about it may be worth connecting one up to see if you
are getting any beep codes. If the board is new it could be that the RAM slots
are a bit snug and not allowing the RAM to seat properly with the force you are
applying. Make sure the RAM you install is properly seated. If it's an old board
that has been sitting out you may want to grab an old toothbrush and some
switch/contact cleaner and clean out the RAM slots. I had an issue where I thought
one of my older Socket 1150 boards had died while sitting on the shelf, turned
out that the RAM slots were just dirty and needed a good clean.