Can I show a relation - "Either in this table or in that " in ER diagram?
In the ER model,  this concept is known as Generalization/Specialization.  A diagram illustrating this is shown in the artcle linked to. 

https://www.geeksforgeeks.org/generalization-specialization-and-aggregation-in-er-model/

A word of caution is in order, however.  Gen/spec is applied to Entities, not tables. The difference between entities and tables is in the level of abstraction.  If you want to think clearly about the subject matter,  and not just the database, it behooves you to keep the levels distinct in your mind.
An entity is an object in the subject matter whose descriptive data may be stored in a table.