假设a和b两个文件行数比例是3:1，不考虑特殊符号：
@echo off
set p=-1
(for /f "delims=" %%i in (a.txt) do (
echo;%%i&set/a n+=1,m=n%%3
set/a test=1/m 2>nul||call :out
))>c.txt
exit
:out
set/a p+=1
if %p% equ 0 (for /f "delims=" %%j in (b.txt) do (echo;%%j&goto eof)) else (
for /f "skip=%p% delims=" %%j in (b.txt) do (echo;%%j&goto eof))
:eof
