In response to your first question,
You could connect two Windows based computers (desktops, laptops, or tablets) to the UD-3900 dock and switch between them by connecting them to a USB switcher such as the Pluggable USB 3.0 sharing switch USB3-SWITCH2. The USB switch device would allow you to switch the dock (and everything connected to it) between two host computers with the press of a button. The DisplayLink driver would need to be installed on both of the host computers first. Then connect and configure the displays on each computer. Afterward, the host computers should remember your monitors’ resolution and orientation so when you switch back and forth you don’t have to reconfigure them.
However, there are some limitations to this setup that you should consider. The USB switcher changes between the two host computers by connecting only one or the other to the shared device, in this case the dock. This means that only the selected computer can communicate with the the dock and the devices attached to it. Changing between computers with the USB sharing switch will disconnect the dock from one computer and connect it to the other. Any USB devices connected to the dock will also be switched with it. Keyboards and mice are not sensitive to being abruptly disconnected, but USB storage devices are, especially if they are in the middle of a data transfer. With that in mind, any USB storage devices attached through the dock should be ejected or confirmed to not be in use prior to switching between computers. This will prevent data corruption by ensuring that there are no active data transfers that could be interrupted.
Additionally, when the dock is switched between computers you will lose your window placement on the monitors attached to the hub. All of the windows on the additional monitors will be consolidated onto the primary display, and unfortunately their positions will not be restored when you switch back.
In response to your second question,
You could add even more monitors by connecting additional USB display adaptors such as the Pluggable UGA-3000 to the UD-3900 dock.
I hope this information helps answer your questions.