At Republic Airways Holdings, our base determines which codeshares we fly for. For instance, when I worked at Shuttle America, my base flew for both United Express and Delta Connection. The pairings (or trips) were built specific to a codeshare. For example, on a four-day trip, as a lineholder, you did not switch between codeshares. The entire trip was flown as either Delta Connection or United Express. (The only time I ever flew both, as a lineholder, in one trip was during the Christmas Blizzard in Denver. I was rescheduled into the Delta system because I couldn't fly my trip for United).
At all three of our airlines, our initial training programs focus mostly on aircraft and safety procedures. Catering and service is taught at the end of training and a lot of the codeshare specific training is received during your Operating Experience, on the line.
The airline is free to reschedule us per our contract, and the flight attendants who would do the most switching back and forth between codeshares in the middle of a trip are reserve flight attendants. Lineholders are not as easily rescheduled, and the pairings we bid on and are awarded are, like I said, codeshare specific.
You can bid your line (schedule) to include multiple codeshares. At Shuttle America I typically bid for one or two Delta Connection four-day trips and several other United Express two-days (those were generally higher credit-hour trips) to allow me to maximize my days off.
From what I know at Mesa, your base also determines which codeshares you fly for, and their trips are built code-share specific, as well.
I hope this helps!
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