Elisabeth Frisorger 

If the termbases have the same structure:

• Save the definition of the new one.
• Export old and new one with default export definition
• Create a new TB based on the stored TB definition file
• Import the master TB first using default import defintion
• Create a new import definition, importing the entries sorted by the source language (the language, which has all entries) and merging existing entries
• Import the old TB using this import defintion

Should the TBs have different structures, I would create a new one from scratch with a structure containing elements from both TBs and then repeat the procedure of exporting and importing TBs.

Elisabeth Frisorger 

If the termbases have the same structure:

• Save the definition of the new one.
• Export old and new one with default export definition
• Create a new TB based on the stored TB definition file
• Import the master TB first using default import defintion
• Create a new import definition, importing the entries sorted by the source language (the language, which has all entries) and merging existing entries
• Import the old TB using this import defintion

Should the TBs have different structures, I would create a new one from scratch with a structure containing elements from both TBs and then repeat the procedure of exporting and importing TBs.

 Jerzy Czopik thanks for sharing that! 

There are different structures.
So, I should create a new one containing all of this fields:

Old structure:

Master, new one:

?

As a result some fields are doubled due to different namings (ex: Status <> Status (Term)) but are used for the same.

My question is, how can I handle that without a lot of manual steps after importing the termbases?

I suppose that I need to open each entry to delete the input of a field...so I'm able to delete the field out of the termbase.

But I cannot save entries if there are mandatory fields without input?