The scenario you outline, where a bad motherboard has fried two CPUs, is a bit far-fetched, but stranger things have happened.
Your testing seems to have shown that the PSU is not the issue, and this is also indicated by the fans starting up correctly. The lack of beeps is the most puzzling and seems to indicate a very early problem with some components, but can also serve as a diagnostic tool. (Hopefully the beeper has not been disconnected by mistake.)
I suggest to do the following :
- Power up without RAM at all
- If you now get beeps, then this is a RAM problem - insert the sticks one by one to locate the bad one, or check their compatibility with the motherboard.
- No beeps mean deep problem with MB or CPU, but the RAM is not involved.
- Follow this tomshardware troubleshooter for further analysis.
Post the results even if nothing conclusive is found:
PERFORM THESE STEPS before posting about POST/boot/no video problems.
If you still cannot find the cause, I would send the motherboard with both CPUs to the motherboard manufacturer and let them know that you suspect that the motherboard has fried both of them before it was repaired. Let them sort it out, and in case the CPUs are fried demand their replacement. Check very well all cables and connectors before sending, to avoid them answering that all is working correctly and the problem is on your side.