I disagree strongly, or rather I will say that your new employer might disagree.

This kind of thing has been tried before, and I know a guy who got fired from a major because he was on vacation from his regional when he started at the major. His intent was to try to protect his seniority in case training went badly.

The problem was that he didn't tell his regional what he was doing...he was basically working another flying job while still employed there, which is an airline no-no.

The major also may have felt that he wasn't sufficiently "committed" to his new job. Whatever.

I've heard of at least one regional which actually has a contract clause which allows a pilot who flunks out of of a major to get his job and seniority back automatically within a certain period of time (a few months, long enough to complete IOE). This is a great thing, and would require no disclosure on your part since it's not an active measure which you took, or fraudulent. It's just kind of there.

But in this case I would either be 100% sure the airline doesn't find out (might be tough since dates of employment are one of the few things companies willingly disclose on reference checks), or make sure both parties know in advance what you're doing and are OK with it.

Otherwise you're taking a chance.