It's not possible to set multiple material and export as a single mesh

I admit my knowledge of obj file spec and how multi-material assignment work in Unity/ProBuilder is limited, but is this possible:

- say I have a cube and assigned 2 materials on different faces, and when I export, the obj file always contain 2 sub mesh (2 "group"), each with assigned material.

- but what if I want to export it as a single mesh (1 "g") but with 2 material (2 "usemtl"), is this possible?

The current way of exporting force creation of sub mesh, I am not sure this is a limit of wavefront file spec, or just an implementation of ProBuilder.

Thx.

EDIT: just tried, Blender can totally export a mesh as a single mesh with multiple materials assigned to different faces, it seems like a ProBuilder limit (I have tried all the export options for obj).