The new artifacts work in multiplayer; I tested them with my brother using Radmin.
The artifacts appear in the artifact shop and in random artifact boxes, unless the map has restrictions on artifacts (restrictions are set by default by the map editor, so they need to be removed in the file).
On RMG/GSC maps, the game never sets restrictions, so the new artifacts appear normally there. Regular maps need to be fixed; I've corrected the maps that I have in the game by removing the artifact restrictions:
