Apple discusses the display support of the 2019 MacBook Pro 16-inch model:
The failure to discuss the huge performance hit of two 5K or 6K displays makes the discussion poorly done in MPG’s view—what user with dual 6K displays can tolerate Thunderbolt 3 speed dropping to USB speed? Such a user surely has substantial I/O bandwidth needs.
Most users needing a 6K display (or dual 5K) that surely need serious I/O bandwidth along with it, although perhaps stock market watchers or gamers might just need a big display. But for them, the Apple Pro Display XDR would be total overkill and the need for dual 6K for anything but video processing or similar seems dubious.
And yet if there is a real need such as with video, using dual 6K would drop both busses to USB speed. Is that realistic for video processing and playback? Maybe, since 1GB/sec per Thunderbolt 3 bus or so is probably still adequate.
Bottom line is that adding a 6K display to a Thunderbolt 3 bus chews up about 30 Gbps of the 40Gbps write bandwidth, reducing the TB3 bus write speed to USB speed (10 Gbps). However, read bandwidth is largely unaffected since the display is a write-sink. That might make it acceptable for some uses most of the time.
If only one 6K display is used, then the other Thunderbolt 3 bus operates on the bus without the display, so that neatly solves the issue for most all uses. Which is why the 2019 iMac 5K is a lousy choice if you plan on getting a 6K display.
Below, recommendations for professional use when a single computer is desired for home and travel: