for batch skip parameter does not support variable delay! n! solution

a.txt

file a line 1
file a line 2
file a line 3

b.txt

file b line 1
file b line 2
file b line 3

merge ab.bat

@echo  off 
REM merge two files line by line into one column 
set n=0
 for /f %%i in (a.txt) do (
 call  :MergeColumn %%i
 set /a n+=1
)
pause
exit /b

:MergeColumn
if %n% leq 0 (set "myskip=") else (set "myskip=skip=%n%")
for /f "%myskip%" %%j in (b.txt) do (echo,%1 %%j & exit /b)
exit /b

operation result:

file a line 1 file b line 1
file a line 2 file b line 2
file a line 3 file b line 3
Please press any key to continue . . .