Other than scheduling your second backup job to be sure it starts the DB2 backup finishes, no there is really no other method
I suggest that you try running your post-exec script by itself to be sure that there is nothing wrong with the script
If not, make sure that the script lives in the same directory as the executables on the cell manager
UNIX
/opt/omni/lbin
Windows
Program Files -> Omniback -> bin
However, I don't see any reason why these backups couldn't be running at the same time, unless they share a device, there is nothing in Data Protector that prohibits this