@echo off 

for %%i IN (c,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t); do call :CHECK %%i  

:CHECK
if exist %1: ( echo exists %1 ) else ( GOTO :FIRST %%1)

:FIRST
if %one == null ( set one=%%1  ) else ( call :SECOND %%1 )

rem mount netdrive on variable one drive

:SECOND
if two == null ( set two=%%1  ) else ( :eof )

rem mount netdrive on variable two drive