An user on The Sims 3 Forums have asked a question about: "If you have multiple sims with 'workable' careers (Careers that you can directly control) what happens to the ones you aren't controlling?"

SimGuru Lyndsay was kind enough to answer this question:

"Several of the active jobs have been designed to allow your Sim to still progress while you're busy with a different Sim elsewhere. For example, a Private Investigator can do low level police work or stakeouts while he isn't working on an active case. The Firefighter is not always fighting fires so there are times where you can let him hang out and socialize at the Fire Station or work out when you aren't controlling him. Both the Stylist and Architectural Design jobs will take you into a mode that will pause the game, but if you don't want to do that you can still have your Sim research their field on the drafting table. Every career has responsibilities and its own active jobs. Each active job will require your full attention, but there are still options that allow your Sim to progress and level up while not controlling them. The most efficient progression will be with you controlling them but the balance is up to you. Similar to how World Adventures worked, it will be a challenge to control more than one Sim with multiple active jobs but we’ve offered many other avenues to help make it easier, and it can be done."