Dual displays on a Raspberry Pi

The Pi 4 and Pi 400 are dual-display capable (they have two micro-HDMI connectors), although some functionality (e.g. H.264 acceleration, HDMI audio, displaying 4K resolutions at 60 Hz instead of 30 Hz) is limited to the primary output only.  Right now TLXOS doesn't have any support for H.265 hardware acceleration, which can be used on both of the Pi 4's outputs.

Unfortunately there is no good solution for dual displays on the Pi 2 and 3:

• USB video adapters using DisplayLink chipsets can be made to work, although this is difficult because you have to use XINERAMA screen aggregation rather than the RANDR aggregation that we normally use (requires manual setup), but even when they do work they perform very poorly.
• The VGA666 adapter that you may have discovered hasn't passed - in all likelihood can't pass - required RF emissions tests, which is why it is only ever sold as a self-assembly kit (it's illegal to sell it fully assembled), which makes it grossly impractical as a large-scale commercial solution. By "self-assembly" I mean using a soldering iron.
• It's not possible to run HDMI and the official 7" touchscreen, or HDMI and Composite, as independent displays. This is a Broadcom firmware limitation.
• Citrix have a screen-casting software solution (Citrix Workspace Hub) that can use a Pi Zero connected via USB to render a second display for your main Pi, but this only works for Citrix HDX.  Unfortunately, they will not permit ThinLinX to distribute this as a software-only solution (only as part of a hardware product), and ThinLinX does not want to sell hardware.