marmoset toolbag can do packing for you. i think you could do same thing in substance painter as well. basically import both models as one fbx. use fill layers to assign the textures that were made for each separate model. export - now multiple textures are combined to a single.
game-ready model of elcan specter sight. the project was made using blender, substance painter and marmoset toolbag. https://www.cgtrader.com/prod https://youtu.be/SS7IsDAPLLE ucts/elcan-specter-docter