Reddit mentions of Plugable Thunderbolt 3 to Dual DisplayPort Output Display Adapter for Thunderbolt 3 Windows Systems (Windows Only, Not Mac Compatible, Supports Two 4K 60Hz Monitors Or One 5K).

You have a couple of options to drive dual 4K monitors.

1. A Thunderbolt 3 Dock like the Dell TB16
   
2. One of these dongles: Plugable or Startech
   
   I've used all three on my 9550 and they all produce dual 4K at 60Hz. As others have said, the HDMI is version 1.4, meaning that it can produce 4K but only at 30Hz.

Wouldn't this allow xps 15 9560 to run two 4k monitors at 60Hz? https://www.amazon.com/gp/product/B01F81EIBC/ref=ox_sc_sfl_title_2?ie=UTF8&psc=1&smid=A3HIHADV23VGU1

Yes. You would need an adapter like this.

This doesn't directly answer your question, but I noticed that the XPS 15 9560 is HDMI 1.4 and that HDMI 1.4 does not support 21:9 aspect ratio...

I try to stay away from the passive USB -> Monitor setups because of problems like the one you're having. I use the Plugable Thunderbolt 3 to Dual DisplayPort and have had great luck so far with dual monitors.
https://www.amazon.com/Plugable-Thunderbolt-DisplayPort-Compatible-Supports/dp/B01F81EIBC

I have a Dell XPS 9550. I had a Dell TB15 Thunderbolt dock that, while great in theory, was nothing but trouble (Dell recalled them).

I recently got one of the adapters below and now have two monitors working very well with Fedora 24.

https://www.amazon.com/Plugable-Thunderbolt-DisplayPort-Supports-4096x2160/dp/B01F81EIBC/ref=sr_1_1?ie=UTF8&qid=1473800032&sr=8-1&keywords=plugable+thunderbolt+3

These adapters are supposed to be able to support dual 4k:
https://www.amazon.com/StarTech-com-Thunderbolt-Monitor-DisplayPort-Adapter/dp/B01ANR4CYE/
https://www.amazon.com/Plugable-Thunderbolt-DisplayPort-Supports-4096x2160/dp/B01F81EIBC/
Also, StarTech has a dock coming out in a couple of months but otherwise have not seen any docks that support dual 4k.
https://www.amazon.com/dp/B01AAAU4WS

In CH too. Getting it from the US personally. Don't like the EU layouts. If you buy from the EU stores you can send back to the EU stores. I think the central point is in the UK as far as I'm aware. If you bought it through the EU the laws require a 2 yr warranty. Not sure about CH laws regarding this though (or if it's part of the EU trade agreements). In the US changing HDD does not void warranty. I'd imagine it's the same here as long as you don't break anything in the process. I will be doing the same thing with the 960. For video I'm going to get this: https://www.amazon.com/gp/product/B01F81EIBC/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

The cable for WD15 can only support 4k @ 30Hz

Same with the HDMI also only 4k @ 30Hz

You would need to get a thunderbolt adapter.

https://www.amazon.com/gp/product/B01F81EIBC/ref=oh_aui_search_detailpage?ie=UTF8&psc=1

Also the TB16 seems to be pretty good in terms of bugginess from what I have read at least.

Defenitely! Use one of those options:

• 1. A Thunderbolt 3 Dock like the Dell TB16
     
• 2. One of these dongles: Plugable or Startech

I trashed my dock and bought an adapter for my displays and am using a USB 3.0 Network adapter (Dell branded) and bluetooth keyboard and mouse as my docking solution...

Here's the display adapter I'm using; works like a champ, didn't even need to install drivers:

https://www.amazon.com/gp/product/B01F81EIBC/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

EDIT: I'm on XPS 15 9550

I'll probably end up using this to drive dual 4Ks at 60HZ

Hmm, none of those sound too easy.

I do own this: https://www.amazon.com/Plugable-Thunderbolt-DisplayPort-Compatible-Supports/dp/B01F81EIBC/ref=cm_cr_arp_d_product_top?ie=UTF8

I use that to drive 2 2k displays at work. I'm wondering if I can test the available bandwidth by hooking it to 2 4k displays. Would that provide sufficient evidence?

I'm not an expert on this, but maybe you could use something like this [Thunderbolt 3 Dual DisplayPort Output Display Adapter] (https://www.amazon.com/Plugable-Thunderbolt-DisplayPort-Compatible-Supports/dp/B01F81EIBC/) if you want to connect two external monitors.

Maybe there are other / better ways of doing it as well.

I've ordered an XPS 9360 and am very concerned about being able to run dual displays using DisplayPort. Has anyone tried Plugable's Thunderbolt DisplayPort adapter? https://www.amazon.com/Plugable-Thunderbolt-DisplayPort-supports-monitors/dp/B01F81EIBC

Sorry for not directly recommending anything, but I feel I can add to the conversation.

I've got the HP Elite 65W TB3 Dock and aside from the horrible coil whine when there's a USB device plugged in (the 90W variant costs the same at the moment), it's an OK dock. If you only need to drive dual displays, you can instead go with the dual displayport adapter which is substantially cheaper. The benefit of the dock would be that it charges your laptop, but I've found it really no worse to plug two cables in instead of one as I need my laptop on full 130W adapter when plugged in.

The UK versions of this seems to claim 60Hz too: https://www.amazon.co.uk/Plugable-Thunderbolt-DisplayPort-Compatible-Supports/dp/B01F81EIBC/ref=sr_1_3?ie=UTF8&qid=1493114845&sr=8-3&keywords=thunderbolt+3+to+dual+hdmi

>Such as? For what price?

According to reviews "Plugable Thunderbolt 3 Dual DisplayPort Output Display Adapter for Thunderbolt 3 Systems" supports two 4K 60Hz monitors & 5K. Customers with XPS 13 (9350) have reported it working also: http://plugable.com/products/tbt3-dp2x/

I'm just interested in the screens, not Wi-Fi, ethernet, sound or other peripherals.

$91.39 on amazon: https://www.amazon.com/Plugable-Thunderbolt-DisplayPort-Supports-4096x2160/dp/B01F81EIBC/

Edit: There's also a StarTech.com version available via accessories.dell.com for $100: http://accessories.us.dell.com/sna/productdetail.aspx?c=us&l=en&s=bsd&cs=111&sku=A8933680

If you don't want to use the other options, you can always just get a TB3 to displayport adapter. I used the dual-display adapter from plugable ($70). I successfully ran one 4k monitor from it with zero problems. There are likely other TB3 to DP adapters you could make use of.

Amazon link for dual adapter: https://www.amazon.com/Plugable-Thunderbolt-DisplayPort-Compatible-Supports/dp/B01F81EIBC

Obviously this doesn't solve the charging requirement.

The other options are the USB-C docks with Displaylink. I'm not personally a fan of this, especially if you are looking for 60hz support. While the device supports 60hz refresh, there is delay between screen writes, so depending on why you want 60hz it might not fulfill your needs.

3rd party charging thunderbolt docks exist, but you may get a nag message on boot that it's not a "Dell" power adapter. This message can be suppressed in the UEFI settings on the laptop.