Can the new 27" iMac Retina 5K display support two SSD's?

"Peter you won't be able to use the Samsung 960 M.2 NVMe SSD. Apple uses a custom PCIe NVMe interface. Besides, the Apple 1 TB & 2 TB units are faster than Samsung's!

You also appear to be looking at the Samsung Pro Series SSD's. Unless you are setting up a dedicated server there is little benefit going to the more expensive Pro line, stick with the EVO line.

FYI - I support over 300 MacBook Pro's which more than 2/3rds have Samsung 850 EVO's. I think we have only replaced one SSD in over three years of use!

As for installing a second SSD, Yes! That can be done as well. But lets first make sure you get the correct model here. You'll want to use the custom configuration selecting the SSD only option with the either the 3.8 GHz or the 4.2 GHz models selecting either the 1TB or 2TB option. Then you'll need to order a SATA HD cable, adapter frame to hold your 2.5"" SSD in the 3.5"" drive bay and lastly the in-line thermal sensor:

Intel iMac 27"" Late 2013 Hard Drive Cable - Apple P/N 923-0312OWC 2.5"" to 3.5"" Drive Adapter BracketOWC In-line Digital Thermal Sensor for Hard Drive Upgrade for 27"" iMacs 2012 and Later

I would just stick with the Samsung 850 EVO SSD Vs the Pro model.

Two last points:

Opening your system will void the warranty, besides the newer 'Thin Series iMac's are a bear to open if you don't use the proper tools and techniques as outlined in the IFIXIT guides. It's quite easy to damage the display which would be a very expensive Opps!

Frankly, I would recommend you hold off on adding the second drive until the warranty period has expired. Instead, I would recommend going with an external Thunderbolt RAID'ed SSD unit as a better direction it will be faster than the SATA internal interface! And then use it for your backup drive."

"Surprising results for 27"" iMac Retina 5K (mid 2017):

TESTED: 2017 iMac 5K SSD Speed vs 2015 iMac 5K (Surprising Finding)

Thus the 2017 iMac 5K is substantially slower for everyday tasks that involve intensive I/O with smaller reads, in spite of the 40% greater speed for large reads.

<https://macperformanceguide.com/blog/201...>

2017 iMac 5K: Flash Drive (SSD)

2015 iMac 5K: writes at 1520 MiB/sec and reads at 2075 MiB/sec

2017 iMac 5K: writes at 2102 MiB/sec and reads at 2915 MiB/sec.

<https://macperformanceguide.com/iMac5K_2...>

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That is less read speed than Samsung 960 PRO with sequential R/W (read/write) speeds up to 3,500/2,100 MB/s and random R/W speeds up to 440/360K IOPS, respectively."