I think so. But remember you won't be able to use the display's speakers, camera, or USB ports. And you have to use an app that transmits DDC/CI to change brightness.
I've only tested the following: Nvidia Titan X, GA-Z170X-Gaming 7, IT-GO "ADP-099-31", 2x GC-ALPINE RIDGE, Startech Thunderbolt 3 to Dual DisplayPort adapter, Dell UP2715K. Some PCIe risers do not work (I tried 1x to 16x) unless you connect them to a computer's PCIe slot. The IT-GO is a 1x to dual 16x and doesn't have that problem.
dnetcrawler was able to use two LG 5K (but one was less than 5K resolution) with a AMD Radeon VEGA Frontier Edition, ASUS X99 Rampage V Edition 10, 2x GC-ALPINE RIDGE (one connected to Thunderbolt add-in card header but this is optional).
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The AMD cards usually only have 3 DisplayPort outputs. To run both displays at 5K, you need 4 DisplayPort outputs. You could supply one of the GC-ALPINE RIDGE cards with DisplayPort outputs from your MacBook Pro using two USB-C to mini DisplayPort cables, but you gain nothing from just using the MacBook Pro's Thunderbolt 3 port instead. If the DisplayPort outputs of the Vega support DisplayPort 1.3, then maybe you can use a DisplayPort 1.3 or 1.4 MST hub to get two DisplayPort 1.2 outputs? I've never seen it and I don't know if it's possible. I don't think such hubs exist yet. A DisplayPort 1.2 MST hub is not sufficient since it doesn't have bandwidth for two 4K@60Hz streams (actually, only two 2560x2880@60Hz streams is required but that's still too much for DisplayPort 1.2).
There are some AMD cards with 4 or 6 DisplayPort outputs. I don't know their price, compatibility, or performance compared to the Vega.
There's no way to convert HDMI 2.0 to DisplayPort 1.2 4K@60Hz yet so that's not an option.