Are you connecting memory for CPU2 when second CPU is installed. You can check with memory in A1 and B1 when both CPU is installed

Can you check whether power of CPU 2 is connected on the motherboard. You can refer Item 20 on below link for CPU 2 power connector location

https://www.dell.com/support/manuals/en-us/poweredge-t420/t420ownersmanual/system-board-jumpers-and-connectors?guid=guid-eefce922-db39-4609-9d94-833d369ac9d3&lang=en-us 

Yes, I have the same memory and same amount in the A bank and B bank

Yes it is connected, while I haven’t tested to see if there is actually voltage at the connector. I’m assuming there is. The E5 2420s work with 2 CPUs installed.

Hello

the E5-2470v2 is not in the list of the supported processor for this system (page 18)

Dell PowerEdge T420 Technical Guide

I think this is the reason of your issue.

Thanks

Forum post 

Post listed above has another list, with the 2470v2 listed as compatible after the 2.9.0 bios update. Would the list be different for dual cpu configs? As in the 2470v2 works in a single cpu configuration but not dual?

Hello would this previous post help?

https://dell.to/3JdexWr

This is to confirm that the Poweredge T420 accepts 2x Intel Xeon E5-2470v2 CPUs with the latest BIOS firmware v2.9.0. 

My system just started and I am looking at 40x Logical processors and 192GB RAM.

I also met challenges with my outdated BIOS. Once the BIOS was updated, everything worked as expected.