It seems to be no longer available. There is no software needed, no configuration, the 2 hdmi screens are presented to the os as one double wide screen.
There seem to be other adapters which support different modes, but they need software support, dont buy those
My test at the time with the Matrox adapter: NO software required!
So the adapter mentioned by @ubu is NOT the only one of its kind that works with Haiku without software - NO trick!
Zotac DisplayPort to Dual HDMI Adapter (zt-mdp2hd) just found used on eBay DE.
I used this adapter from MATROX with Haiku back then - WITHOUT software.
It worked perfectly with my hardware configuration at the time and out of the box.
Uhh, I remembered so, it was about some dual monitor solution was getting out of a conververter HW, but forgot about details :
it is splitting a display to two half area,
not at least a mirroring … I would like to have…
It would be for using a projector, besides my laptop main window.
Anyway – nice for some applicable use cases, as big big workspace area that expanded on two similar monitors in a dual setup.
Setting Deskbar’s place to right or left basically solve that :
it is splitting the Deskbar itself also on the displays,
if that resides on bottom or top of Haiku desktop.