Thank you for posting the question!
The short answer is, I think the UD-3900 docking station and the USB3-Switch2 switch can do the job you are looking for, and I guess it’s one of the most cost effective way.
But you’d need an additional port adapter, and there are a couple of notes.
I’ve just quickly checked the specifications of the Dell UltraSharp U2412M monitor then found it has a DP, DVI-D and VGA ports. The optimal resolution is 1920 x 1200 @ 60 hz, which is totally fine with the UD-3900.
UD-3900 docking station has two video ports: one DVI-D and one HDMI. In order to connect two Dell U2412M monitors, you’d need one HDMI to DVI port adapter to convert the HDMI port of the dock.
Since there is very limited spaces is left on the back of the docking station, I’d recommend using an adapter cable such as the following, but not a dongle.
Here is the couple of notes:
Please note that the UD-3900 docking station comes with 5V4A power adapter. The total power drawn from the docking station itself and the connected peripherals cannot exceed the power adapter can provide.
This means if you connected multiple bus-powered and high-power (which will draw up to 900mA per port) USB devices to the dock, the power shortage could occur. If you notice any unstable state of the USB devices, you may need to move some USB peripherals to other USB ports of the system or the monitors, in order to resolve the power shortage.
It doesn’t seem you’d connect any storage devices (i.e. HDD) to the dock, but if that the case, switching it between two systems may cause an issue. Plugable USB Sharing Switch is basically to do unplugging an peripheral from one system, then plugging it to another. We don’t recommend to use storage devices with a sharing switch because of the possible data corruption.
Please find more details from the product page:
If you thought the #1 of above could be a problem, you could use a high power USB hub plus additional USB devices (such as two USB graphics adapters, one USB Ethernet adapter, and so on). For example Plugable has a USB 3.0 7-port hub that comes with 12V5A power adapter.
But the total cost of that configuration would be way more expensive than the UD-3900 + the HDMI to DVI adapter cable.
I hope the above information helps!
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Thanks again for posting this question here!