1. For two of those processors you only need two of the 463990-001. That is the official HP "Mainstream" heatsink/fan for Z600 (and Z800) workstations that can properly cool 95W max TDP or lower processors. If you already have two of those heatsinks/fans you are set.... unless you choose to go for hotter processors than 95W max TDP. Reportedly the Z600 version 2 can run two X5690 processors, 130W max TDP, but those are quite expensive and need to run under two of the Performance heatsinks/fans. In that case you could get by with one official Z600 Performance heatsink/fan, and also use one Z400 Performance heatsink/fan (detailed in the link in #2 below). You could also have your second heatsink be an official Z600 Performance one too. That combination would cost quite a lot.
2. As I explained HERE, the "Performance" heatsinks/fans are meant for processors that run hotter than 95W max TDP (usually 130W max TDP, for these workstations). A Z400 Performance one is part number 463981-001, and will fit on only 1 of the 2 sockets for the Z600, with a little modification of the angle of the rear bottom thin metal airflow deflector.
3. If your Z600 is a "version 2" with the later boot block date you can do very much better in your choice of processors, as explained in the link in #2 above. You can read up in this forum on what the Boot Block Date is for a version 1 versus a version 2... that date is easy for you to find from your BIOS.
4. You need to aim higher than the two processors you mention, if you have a version 2 Z600. There are much better options for you in that case.
Good luck on your project! That workstation can really be souped up very nicely.