Try this,
set SOLUTION_PATH="%~dp0\..\..\VSSolution\Customer2013.sln"
set SOLUTION_PATH1="%~dp0\..\..\..\..\Customer1\Customer1.sln"
set mode=%1
echo %REL_PATH%
if /I "%mode%" == "release" (
set value="Release"
set filter="*64.dll"
) else (
set value="Debug"
set filter="*64d.dll"
)
echo "Build Mode set as %value%"
echo "Starting Debug Build for Customer solution"
devenv %SOLUTION_PATH% /rebuild %value%
echo "Starting Debug Build for Customer1 solution"
devenv %SOLUTION_PATH1% /rebuild %value%
echo "Build is completed"