If we assume they started both on the same date, then they wouldn't have had to lie or mislead prior to that point.
Then the question is only whether it's legal (contractually) to be employed by more than one employer at a time. But to that... would it be legal for a company to own every waking hour rather than just your employed hours?
I've not seen anything that says someone has to be exclusively hired, but they would need to be able to fully do their job for their employed time and surely that's all that matters.
I'm in the top 25% compensation for senior SWEs in my area, according to levels.fyi, and I only have 5 YOE, so I guess so. I don't know that either me or them feel like that provision is the driver of this, but if it weren't a great offer I'd probably fight harder.
Then the question is only whether it's legal (contractually) to be employed by more than one employer at a time. But to that... would it be legal for a company to own every waking hour rather than just your employed hours?
I've not seen anything that says someone has to be exclusively hired, but they would need to be able to fully do their job for their employed time and surely that's all that matters.