Portable SLI/CrossfireX external box … gimme gimme!
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.