I can't speak from experience, but my bet would be this:
It does seem uncommon to have multiple jobs in a year, unless you were somehow able to separate both on your duty time, and fulfill rest req's. If you left because of an accident/incident/safety concern, there would be a problem.
HOWEVER, as I understand, 121 ops are looking for pilots who will be happy in their airplanes. If you spent 6 months or less at a job, and it wasn't the job you thought it was, you will probably have the opportunity to explain that in an interview.