--run against instance 1
EXEC xp_cmdshell 'SQLCMD -S"Instance1" -E -dmaster -i"\\path\01a_update_Instance1.sql" -o"\\path\script_output_Instance1.txt"'

--run against instance 2
EXEC xp_cmdshell 'SQLCMD -S"Instance2" -E -dmaster -i"\\path\02a_update_Instance2.sql" -o"\\path\script_output_Instance2.txt"'
GO

--run against instance 1
EXEC xp_cmdshell 'SQLCMD -S"Instance1" -E -d"DB1" -i"\\path\update_stored_proc.sql" -o"\\path\script_output_Instance1.txt"'

--run against instance 2
EXEC xp_cmdshell 'SQLCMD -S"Instance2" -E -d"DB2" -i"\\path\update_stored_proc.sql" -o"\\path\script_output_Instance2.txt"'
GO