There are ways to do this, but the details will vary depending on what you need.
Off hand, I'd set up a "master job" table where one record always = one job. Your existing jobs table would become "sub jobs" linked to a serial ID field in the new table. For most of your jobs, you'd have one sub job linked to one Master Job table, but whenever you get the situation you describe, you'd link all your sub job records to the same Master Job table so that they can be treated like one combined job for billing, status and other purposes.
This would be quite a bit like setting up an invoice where the invoice has a portal to a line items table for listing each item purchased on that invoice.
Thanks, I'll give it a try.