I don't know of any adapters that can suppport 2 SSD's via PCIe bus. However you can always use 2 adapters with 2 SSD's.
Just make sure you setup your primary boot drive in the system bios to boot from the SSD first.
If you get an M.2 SSD then I know that the Samsung Evo 850 and Samsung SM951 SSD will work as they both are "ACHI".
Advanced Host Controller Interface (AHCI) is a technical standard defined by Intel that specifies the operation of Serial ATA (SATA) host bus adapters in a non-implementation-specific manner.
I don't know if the HP z800 will support the NEWer "Nvme" SSD's as in the Samsung 950 Pro or Intel 750 PCIe SSD.