My crude solution would be to add a comma after the first job title, then type in the second job title.
This, however, does not resolve the issue if one of your contacts has multiple job titles at multiple employers.
In Address Book, Card > Add Field > Job Title gets greyed out after you add just one job.
I like your question and it's a great idea for an improvement to the Address Book.
An alternate solution would be to ask your friend to quit their second job. This gives them more free time to spend here on everythingiCafé.