Here's a pure batch solution, dull, I know, but it is what the OP asked for.
@echo off
setlocal enabledelayedexpansion
set file1="file1.txt"
set file2="file2.txt"
REM count lines
REM presumes both files have same number of lines
for /f "tokens=1,* delims=:" %%L in ('findstr /N /R ".*" %file1%') do set /a lines1=%%L
for /L %%N in (1,1,%lines1%) do (
set line=%%N
set linecount=1
for /f "delims==" %%L in ('type %file1%') do (
if !linecount! equ !line! set file1line=%%L
set /a linecount+=1
)
set linecount=1
for /f "delims==" %%L in ('type %file2%') do (
if !linecount! equ !line! set file2line=%%L
set /a linecount+=1
)
FC "!file1line!" "!file2line!"
)
2 files
file1.txt file2.txt
file1-001.def file2-001.abc
file1-002.def file2-002.abc
file1-003.def file2-003.abc
file1-004.def file2-004.abc
file1-005.def file2-005.abc
file1-006.def file2-006.abc
file1-007.def file2-007.abc
file1-008.def file2-008.abc
file1-009.def file2-009.abc
file1-010.def file2-010.abc
file1-011.def file2-011.abc
file1-012.def file2-012.abc
file1-013.def file2-013.abc
file1-014.def file2-014.abc
file1-015.def file2-015.abc
file1-016.def file2-016.abc
file1-017.def file2-017.abc
file1-018.def file2-018.abc
file1-019.def file2-019.abc
file1-020.def file2-020.abc
file1-021.def file2-021.abc
file1-022.def file2-022.abc
file1-023.def file2-023.abc
file1-024.def file2-024.abc
file1-025.def file2-025.abc
Output:
FC "file1-001.def" "file2-001.abc"
FC "file1-002.def" "file2-002.abc"
FC "file1-003.def" "file2-003.abc"
FC "file1-004.def" "file2-004.abc"
FC "file1-005.def" "file2-005.abc"
FC "file1-006.def" "file2-006.abc"
FC "file1-007.def" "file2-007.abc"
FC "file1-008.def" "file2-008.abc"
FC "file1-009.def" "file2-009.abc"
FC "file1-010.def" "file2-010.abc"
FC "file1-011.def" "file2-011.abc"
FC "file1-012.def" "file2-012.abc"
FC "file1-013.def" "file2-013.abc"
FC "file1-014.def" "file2-014.abc"
FC "file1-015.def" "file2-015.abc"
FC "file1-016.def" "file2-016.abc"
FC "file1-017.def" "file2-017.abc"
FC "file1-018.def" "file2-018.abc"
FC "file1-019.def" "file2-019.abc"
FC "file1-020.def" "file2-020.abc"
FC "file1-021.def" "file2-021.abc"
FC "file1-022.def" "file2-022.abc"
FC "file1-023.def" "file2-023.abc"
FC "file1-024.def" "file2-024.abc"
FC "file1-025.def" "file2-025.abc"