I have a number of quite sophisticated scripts for maintenance with MP3Tag.
Some of them have more than 50 function calls and are quite complex.
Due to a change in my workflow, I am now faced with the task of turning two scripts into one.
How do I get them into an MTA file for further processing? 
You could merge the various MTA in one new (or existing) MTA.
Just open them in a text editor and copy & paste the content.
You would have to adjust the numbers like
[#1]
for every action block to be sure that you don't have numbers twice.
BUT
I would think about this step carefully!
Every additional complexity makes the debugging for errors more costly in time and effort.
Some steps may only work in a certain order and even "destroy" some ideas that have been executed before or follow later.
Be extremely careful and test your new merged script with all possible use cases.
Thank you very much for pointing this out. 
I am very well aware of the problem of sequential numbering.
Is there a way to avoid these pitfalls (and the manual work)?
Unfortunately I don't see any easy and automated way.
The other way around:
Why do you think that merging two scripts toghether will have any benefit for you?
(Besides the obvious few mouse clicks you may save).
The motivation to try this is that about 20 functions in both scripts are redundant, but more than 30 tested functions together should form an important building block for my future workflow.
There is a free Windows tool called WinMerge.
This tool let you load 2 different files and then compare them and show you the differences.
You can decide if you want to move a difference from the left side to the right side and vice versa.
Maybe this tool can help you to merge the relevant parts into one file.
This is also not an automated way, only a visual help.
