Just an update for future reference. We solved our problem by going with Dell's AMD models instead. AMD CPUs have more PCIe lanes so they can support more NVMe drives and I/O in general on just one CPU. The R6515 and the R7515 are the ones we looked at. The R6515 would be the equivalent of the R640 and the R7515 the equivalent of R740xd. We went with R6515 model (1U) in the end as it had two extra PCIe slots as well as up to 10 NVMe/SAS/SATA drives using just the single CPU. The largest CPU that fits in this small server has 64 cores.  
I'm looking for a server that can take one scalable CPU (for now) and has support for NVMe drive bays.   I checked R640 and R740xd but online configuration says that you need two CPUs to have NVMe support.   Are all the NVMe drives really just connected to CPU2 on the motherboard? Or is this a sales thing and not a technical limitation?  
R640 or R740xd with NVMe drive bays and one CPU?