There are USB 3.x male to male cables that should work for you. Here's one example. -> https://www.startech.com/Networking-IO/USB-PS2/usb-file-transfer-cable-windows-mac~USB3LINK
I have not used such a cable myself but there are several such cables on the market and it appears that they all use the same chip inside. It sounds like they can be put in a "serial mode" or an "Ethernet mode". Each mode is much like it sounds, a serial mode will look like having two USB to serial adapters back to back, and Ethernet mode will look like two Ethernet adapters back to back. Cables like this are generally preferable to pairing up serial or Ethernet adapters because this is faster, 5 gigabits instead of 100 megabits per second, and is cheaper, because if someone were to track down a couple gigabit Ethernet adapters the cost would be far higher.
Take great care in selecting cables because there are still many USB 2.0 devices like this which will only get you 480 Mbps. Even worse are a number of cables that do not comply with the USB standard and will SHORT OUT THE POWER PINS!
There are passive, and USB 3.x compliant, crossover cables (compliant because they do not connect the power pins) that could work if someone could find drivers. Cables like this -> https://www.datapro.net/products/usb-3-0-super-speed-a-a-debugging-cable.html
Both laptops do not have to be connected by the same means. A Thunderbolt cable between the Mac Mini and one laptop will create a very fast network, up to 20 Gbps with Thunderbolt 2. Cables like this -> https://eshop.macsales.com/shop/Thunderbolt/OWC/Thunderbolt-Cables
Connecting everything by Ethernet is a "safe" choice as setting up a network this way is quite simple, low cost, well documented, etc. All you need is a couple Thunderbolt to Ethernet adapters and a hub, or three Ethernet adapters (one for each computer, plus using the one Ethernet port already in the Mac Mini) and connect the two laptops to the Mini by Ethernet. Thunderbolt to Ethernet adapters like these should do -> https://eshop.macsales.com/item/Apple/MD463LLAU/
Here's what I'd do in this case, Thunderbolt to Thunderbolt from Mini to one laptop and the USB 3.x transfer cable to the other. The Thunderbolt connection should practically set itself up, it's things like this that the port was made for. The USB transfer cable is not something I've tried before but the people that make them say they work. If not then return it and get a gigabit Ethernet adapter instead.
The reason why the dual setup isn't working is that you are using MiniDP & HDMI when dual monitors via MiniDP and HDMI are not supported, as noted in the product guide. You will have to use a Thunderbolt-ready monitor with a Thunderbolt cable to allow both monitors to be driven from the dock.
The reason for this is that your dock is being driven by one Thunderbolt port, which does not allow for daisy-chaining displays (as far as the port on your computer is concerned) unless it is Thunderbolt 2 or the newest revision of DisplayPort. If you were to plug your monitor either into the spare Thunderbolt port on your computer (HDMI on the dock) or use the HDMI port on the computer (MiniDP on the dock) both monitors should work as a workaround.
Best Answer
Theoretically, yes you can do this. You would need a Mac with multiple Thunderbolt ports like a 2013 MacPro with its 6 Thunderbolt ports. Not sure if daisy chaining multiple Macs together will give you the desired effect.
This link seems to think that it will work but says nothing about daisy chaining only plugging a bunch of Macs into one Mac with a lot of ports.
So it seems that it is safe to plug a bunch of Macs together VIA thunderbolt which opens the door to you experimenting with various topologies to find one that does what you want.