You can with cheats. careers.add_career {careername}
The code names for the careers are listed here: http://sims-online.com/sims-4-game-info/sims-4-game-cheats-codes/
You would have to make sure the hours didn't conflict. The teen jobs for everyone mod is useful for giving second jobs to sims with a daytime career already. (Note: I haven't actually tested this in my game yet any further than actually assigning the jobs, which does work, but I have seen other people play through and say that it works. ETA: I just tested through a whole work session with both jobs and it was fine, got paid for both, got progress for both. The only problem is the game only seems to store one work outfit, so they'll wear the outfit for the newest job to both jobs.)
