The Elder Oblex has the Sulfurous Impersonation feature which lets it create an impersonation of a creature in another space. The feature goes on to state:
For all practical purposes, the simulacrum is the oblex, meaning the oblex occupies its space the simulacrum's space simultaneously.
In the likely event that the Oblex has multiple simulacrums on a battlefield, what happens if 2 or more of them are subjected to an area of effect spell like fireball? Does the answer change for a wall spell like Blade Barrier?