aloha20200628

@echo off & setlocal EnableDelayedExpansion
findstr /n .* a.txt > a.b
findstr /n .* b.txt >> a.b
set/a n=0
(for /f "delims=" %%s in ('sort a.b') do (
  set "z=%%s" & set "z=!z:*:=!"
  if !n! equ 1 (set/a n=0 & echo,!ls!!z!) else (set/a n+=1 & set ls=!z!)
))>ab.txt
del/q a.b

aloha20200628

再精简一步：
@echo off & setlocal EnableDelayedExpansion
findstr /n .* a.txt > a.b
findstr /n .* b.txt >> a.b
set/a n=0
(for /f "tokens=1* delims=:" %%1 in ('sort a.b') do (
      if !n! equ 1 (set/a n=0 & echo,!ls!%%2) else (set/a n+=1 & set ls=%%2)
))>ab.txt
del/q a.b