Yes, I figured it out

. I'm sorry, I'm a new SP user, so I wasn't aware that GLB / GLTF didn't support UDIMs at first.
Thanks for the info!
Now, I have a new issue.. whenver I export to GLB / GLTF, and try opening it Win10 standard 3D viewer, it looks all black. However, when using an online GLTF viewer, it looks fine.
Ideas?