The Node Duo, with space enough for two half-length PCIe cards, is the ultimate plug and play expansion kit for Thunderbolt 3 computers (e.g. Laptop computers) where installing an internal expansion card is not an option and for systems where there is not enough space for more PCIe cards.
In the unlikely event that you need to lug two graphics cards for an on-site job (think having to carry an ultraportable with two professional graphics cards for touch-ups and rendering on the fly), AKiTiO has a solution, with the Node Duo. This box measures about 29.3 cm x 15.5 cm x 23.2 cm (DxWxH), and encloses two PCI-Express 3.0 x16 slots (electrical gen 3.0 x2), which it wires out upstream with a Thunderbolt 3 (40 Gbps) connection. Each slot has room for 2-slot thick graphics cards that are up to 22 cm in length. The enclosure ventilates them with a 120 mm intake fan.The Node Duo uses USB-C ports with Thunderbolt 3 wiring. The upstream port has standard 15W delivery, and plugs into your PC; whereas the downstream port supports USB PD, with up to 60W downstream power delivery. This port has no USB link-layer, only Thunderbolt.
There's also a standard-size DisplayPort 1.2 output, if your Thunderbolt input is meant to daisy-chain with a DP monitor at one end (not with display from the installed graphics cards, though). Out of the box, you get a 150W power-brick that supplies up to 25W each to the graphics cards through the slot; although you can expand that to a 230W PSU that supplies up to 75W. Available from a little later this month, the Node Duo by AKiTiO is priced at USD $394.95.