There are many way to make modular stuff.
I just expose a simple basic one in Blender, it just covers modeling and preparing for FBX export.
Mirror on Y the object
Align to grid to be sure it is aligned on red axis with the other
For the UV part :
Just download the Blender file before the merging , make UV seams on one half modular piece , this will be automatically made on all instances.
I hope you enjoyed this other Blender tutorial