Interleave files with CMD using echo

Question

How could I interleave files line by line with

cmd.exe

with help of read, grep, echo, etc?

File1.txt with the context:

Answer 1

You need a method to read from two files in parallel. This is possible by using two methods at the same time (<file1 set /p and for /f ... in (file2)):

@echo off
setlocal enabledelayedexpansion

<file2.txt (
  for /f "delims=" %%a in (file1.txt) do (
    set /p b=
    echo %%a
    echo !b!
  )
) >outcome.txt