These are examples of PCIe add-on cards that could work for software raid of the boot drive. Booting from M.2 SATA is much more likely to work with old hardware than NVMe. XCP-ng installer may automatically make a software RAID out of them. Most of the other dual M.2 cards are funky in that one is M.2 NVMe and the other M.2 SATA.
$62.00 dual M.2 SATA only with no need for SATA cables. StarTech.com 2x M.2 SATA SSD Controller Card - PCIe - PCI Express M.2 SATA III Controller - NGFF Card Adapter (PEX2M2), Red $43.00 four port NVMe only PCIe card. GLOTRENDS PA41 Quad M.2 NVMe to PCIe 4.0 X16 Adapter Without PCIe Bifurcation Function, Support 22110/2280/2260/2242/2230 Size (PCIe Bifurcation Motherboard is Required)