Dual/Quad socket LGA 2011 Servers that officially support two or more server GPUs
- Superserver 2027GR series (can be found on ebay)
- Superserver 4027GR series ($$$ on ebay)
- DL380p Gen8, correct riser + cable kit + PSU options required (see page 50 of quick specs), https://www.hpe.com/h20195/v2/GetDocument.aspx?docname=c04128241
- DL580 Gen8, cable kit required (see page 30 of quick specs), will also support 2 GPUs that need 2 power connectors, https://www.hpe.com/h20195/v2/GetPDF.aspx/c04199329.pdf
- R720 (not xd), correct riser required. http://www.dell.com/support/manuals/ed/en/edbsdt1/poweredge-r720/720720XDOM-v3/GPU-Card-Installation-Guidelines?guid=GUID-268578E4-B96B-43BA-AC13-58C567CBB670&lang=en-us
Be warned rackmount servers from these companies expect server GPUs which always have the power connectors on the end not the top like almost all consumer GPUs, typically there is no room to use top connectors. If you 100% need to use top connectors the HP DL580 Gen8 and Supermicro 4027GR are the most likely to work, both either not on ebay or costly.
Personally I think your best option is to buy a Supermicro rackmount case that has the correct PSU option for GPUs and populate it yourself using the best motherboard deal you can find.