I've two m.2 drives that I want to install on my new build, but the mobo only has one m.2 slot. I am thinking of buying a m.2 to PCIe adapter, but will this slow down the drive in any way?
No, but it may slow your graphics card. It could cause the graphics card to run as an x8 device. For gaming it should not be noticeable. Rendering might be impacted.
Are the M.2 SSDs using M.2 SATA or M.2 NVMe? What's the brand and model of the motherboard? Which PCI-E slot on the motherboard will the PCI-E add-in card be inserted into?