I'm getting one at 3840x2160@59Hz and one at 1920x1080@59Hz. Ideally I would like both at UHD, but I can't seem to find out if it is actually supported.
Speaking strictly from the standpoint of the Intel UHD Graphics 630 engine, it can support three independent DisplayPort (DP) 1.2-compliant output streams. Each of these output streams can support single- or multiple-display configurations, but, overall, the Intel UHD Graphics 630 engine supports a maximum of three displays.
In the SP6 design, only two of these DP output streams are actually utilized. One is used to support the built-in display and one is exposed, via a mini-DisplayPort (mDP) connector, for external display connections. With an adapter that includes an MST Hub, two monitors can be connected to this mDP connector, but it needs to be understood that these monitors will share the throughput capabilities of the (single) DP output stream. This will be the case using a separate adapter or using the Surface Dock.
Here is an example of a mDP MST Hub with DP outputs that is certified for MS Surface: https://www.amazon.com/Accell-Displayport-DisplayPort-Multi-Display-Splitter/dp/B00OJZSOLE. I do not know why this adapter needs additional power (provided via USB power connector); I have seen others, with both DP and HDMI outputs, that do not require this USB power connector. This may be limitation of the MS Surface design.
A DP 1.2 output stream will support a single UHD display at 60Hz. If you add a MST Hub, it is only possible to support two UHD displays at 30Hz. You might be able to run one UHD display at 60Hz and one FHD or WUXGA display at 60Hz, however. I say 'might' because, while the math would seem to work out, I have not tried this. I will try using a MST Hub in this configuration and let you know how it goes.
Hope this helps,
Thank you, that gives me a lot more information than I understood before. If I did understand you correctly, it sounds like my only option may be to use a USB 3.0 dock and take the frame rate hit to drive both displays, as I do not believe my monitors will support a 30Hz refresh rate. I have a colleague that does have a MST hub, so I'll try that tomorrow to see if I have any luck just in case.
Ok, I tried this on my NUC8i7BEH NUC. It has a Core i7-8559U processor, which actually has the Intel Iris Plus Graphics 655 engine, but this is similar in capabilities and performance to Intel UHD Graphics 630. This NUC has a Thunderbolt 3-compliant USB-C connector that exposes one of the DisplayPort 1.2 output streams. I used a MST Hub that provides a USB-C-to-2xHDMI connection. I connected my main display (actually a Samsung 40" 4K TV) plus a secondary display (a Seiki 22" FHD TV).
I was NOT able to connect both displays at 60Hz. I was, however, able to connect both displays at 59Hz (go figure). Oddly though, if I disconnect and reconnect the 4K display (which I do regularly as I have a 4-port 4K HDMI KVM connecting this 4K display to three NUCs as well as my main PC), the connection will always drop back to 29Hz. I an unsure how I can lock this at 59Hz all the time - but I will figure it out... offline 😎
I was able to test out my colleague's MST adapter on two 2560x1440 displays and all was good. I'm going to see if that resolution works on my monitors at home as well. I'ts still not full resolution, but it might just be enough. Thanks for all of your assistance on this.
Really strange thing happened today. My laptop froze so I power cycled it. When it came up, still connected to the dock, the display that was previously set to 3840x2160 was only able to switch between 1024x768 and 800x600, while the other remained at 1920x1080. I disconnected and reconnected mini DP and dock cables, and tried rebooting the laptop, but no change. Finally, I pulled the cable for the low resolution display and plugged it directly into the laptop. This changed the display setting to duplicate display across both at 1920x1080. I switched to extend display and then changed the resolution to 2560x1400, but for some reason the output on both displays switched to full 3840x2160 at 59Hz. Of course I'm afraid to touch anything right now.
So, it is absolutely possible to drive two full UHD monitors from a Surface Pro 6 with the Intel(R) Graphics UHD 620. However, I wish anyone good luck trying create the right conditions to do it.
I am not sure that I followed you completely, but these displays must somehow be using different DisplayPort streams; that is the only thing that makes sense. It is physically impossible to support two 4K displays at 59/60Hz on one DisplayPort stream. Per the DisplayPort spec (see summary tables in Wikipedia), it requires 12.54Gb/s to support one 4K Display at 60Hz. The total bandwidth of the DisplayPort stream is 17.28 Gb/s, so no way can two 4K displays be supported at 59/60Hz.