It is possible to use multiple UVs with Arnold. The image above has a tiled brick texture overlayed with a single graffiti texture.
This workflow is obsolete, multiple UVs are now translated automatically if they are named uv2, uv3, etc.
To use multiple UV sets in Arnold, proceed as normal in Houdini but eventually, the UV set attributes need to be converted to vertex float attributes of size 2 in order to translate to Arnold POINT2 userdata.
When using attribcreate to add a UV set it must be of class Point, size 2 with $UV1 and $UV2 as the values.
After creation, the uvsets must be promoted from point to vertex attribute to be converted properly to indexed POINT2 userdata in Arnold.
The UV sets must also be explicitly flagged for export in the Arnold properties of the geometry object.
The UV set is assigned in the shading network by using the UV Set parameter of the image node.