There are different possibilities.
1. Can you change JOB A to do the SDMJOB for JOB b?
2. Does JOB A do something unique?
Like write records to a file that is normally empty. If so, Add logic in JOB B to first see if there are records in the file and also make sure there is no lock on the file. This should mean JOB A completed normally. JOB B can continue. If no records in file ot if you cannot ALCOBJ, then you can either do a DLYJOB for x period of time, then loop to see if condition has changed and this job can run. It would continue to loop until JOB A is completed.
There are otehr methods, but these are the easist I know.