I have known a number of pilots who went elsewhere, and remained on a seniority list, from a given operator. To say it's not done is untrue. To call it "selfish" is ignorant and judgemental.

I have known operators who didn't pay out sick time when the employee left; the employee with sick time and vacation accrued might get nothing if he didn't take it. It's his time, after all, and taking it at the end of a hitch, whether sitting home, out fishing, or off at employer B, it's still his time. Selfish? Hardly. Selfish and dirty is the company that withholds that time from the pilot.

As for how the next employer views "dual employment," that's up to the employer, if the employer knows at all. I'm aware of pilots now who are on a seniority list and yet working elsewhere, and both companies know.

In leaner times, it was quite common for furloughed pilots to go to company B and be told they'd need to resign their seniority, and yet company A understood, tongue in cheek, and tore up the resignation, knowing that the pilot would be back. Everyone knew, really, given that times don't always stay lean.

The easiest way to handle the move between companies is to have a gap between leaving one and hiring at the other, to make the move, but given that you typically wind up at a hotel with your class for a couple of months, and you wouldn't be home anyway, its often just as easy to go to the new company, get hired, go to class, and move after. Or commute for a while and move at your liesure. In the airline world, that's what people do; commute. For ACMI carriers that are home-based and buy you a ticket to work, moving for work becomes a memory. For carriers in which you might enjoy a couple of weeks off at a time, moving during your days off might be the better option.