The online manuals are here.
If you order the Aurora-R8 with a single (dual-slot) Nvidia card like a RTX-2080, you will still have:
One PCIe-x16 slot (x8 electrical)
One PCIe-x4 slot
So, no problems installing another PCIe card in one of those lower slots.
Aurora R8 Setup and Specifications PDF, pages 14/15 =
Aurora R8 Service Manual PDF, page 14 =