Dual CPU trays are still extremely expensive to buy, check eBay prices, you will be surprised.
Maybe it's better, and less total cost in the end, to find a working dual CPU Mac Pro.
Btw, if you never repaired a Mac Pro, nor have another working to test the components from the non-working one, don't buy a non working one.
Yeah, that was my point. If the OP is hoping to actually use this machine, it sounds like they are building a thee story mansion, on the end of a thin tree branch.
There ARE other alternatives, especially if they are thinking of replacing the CPU tray. I mean, that's EXPENSIVE. That's like buying a junker BMW and thinking 'well, I was going to buy a new motor, transmission, and interior. Sounds simple, right? Um...
I looked on ebay, and saw A Whole Bunch Of Mac Pros that were WORKING! And many had DUAL CPU'S that also work, and WITH memory.
Buy that box, and who knows what you might have. But it's your money.