Looking at different motherboards one can notice that they can be distinguished by some of these terms: ATX, mATX etc. This pretends to the size of the motherboard. My question is what are the motherboards that can hold 2 CPUs called in technical jargon?
I don't think there really is one.
Dual Socket or Multi-Socket is a description rather than a specific term. With multi-core being the norm multi-socket motherboards have become something only used in servers (and a few very expensive workstations that are really servers in a more user friendly case and the ability to add graphics cards1. Today entry level servers are losing their multiple sockets—one CPU being enough for basic workloads.
Even before multi-core, even dual-socket motherboards were a high end and and specialist and never entered the mainstream2
1 However this is also becoming the norm for servers with dedicated GPU accelerators being added to servers for HPC usage.
2 And I certainly include the workstation box now mostly retired sitting here, powered down, retained just in case I need anything on it.
In 2010, EVGA Corporation released a new motherboard, the "Super Record 2", or SR-2, whose size surpasses that of the "EVGA X58 Classified 4-Way SLI". The new board is designed to accommodate two Dual QPI LGA1366 socket CPUs (e.g. Intel Xeon), similar to that of the Intel Skulltrail motherboard that could accommodate two Intel Core 2 Quad processors, and has a total of seven PCI-E slots and 12 DDR3 RAM slots. The new form factor is dubbed "HPTX", and is 13.6 by 15 inches (34.5 cm by 38.1 cm).