@echo off
setlocal enabledelayedexpansion
rem m表示几行合并为一行
set /a m=2
for /f "tokens=*" %%i in (test.txt) do (
  set /a hangn+=1
  set /a n+=1
  set line=!line!%%i
  if !n! equ !m! (
    echo !line!>>result.txt
    set line=
    set /a n=0
  )
)
rem 以下代码用于处理剩下的行
set /a t=!hangn!%%!m!
if !t! equ 0 goto end
echo !line!>>result.txt
:end
endlocal