Only 1 Monitor works at a time with Dual USB Monitors

[Stepinsky]

I had the same issue when running my Lenovo LT1421 (USB2) together with an Asus 168+ (USB3). When I connect both monitors to my ThinkPad,
only one monitor is working at a time. It was not a power issue. I verified that with a self powered USB hub.
Both monitors were running without any problems when my ThinkPad was sitting in the original docking station, which has an additional build-in USB hub. I was running the LT1421 from USB ports at the back of the docking station and my Asus was connected directly to one of the USB3 ports of my notebook.
Running two of the Asus 168+ directly from my notebook is working flawlessly.

So I suppose it's an USB hub or bridge issue, maybe related to different USB modes dependent on what you connect. If both monitors are sharing the same hub, they're interfering. If you put them on different hubs or bridges, they cooperate. Or it's a driver issue with the display link drivers and older chip models.
I couldn't figure it out and so I'm using the dual Asus setup when working on the road and having the need for a three screen setup.