People work multiple concurrent jobs all the time. Testament to this is the fact that the Form W-4, which you have to fill out whenever you start a new job, has section 2 titled "Multiple Jobs or Spouse Works".
But more than that, what do you think happens if you worked one job from Jan to Jun and then another job from Jun to Dec? In that scenario you'd get 2x W-2's. Since W-2's don't say the date range of your employment that means that, in-so-far as the IRS knows, you did work these two jobs concurrently
More importantly, do you have to declare with each of the employers that you hold another W2 job (with a different employer)? For example, I work on W2 job with employer A and have just started working on W2 with employer B. Do I have to declare at employer A that I hold another W2 job and vice-versa?
It might not be a bad idea to tell both employers about the other job you're working for, depending on your situation, but it's not a legal requirement. That said, I'd double check my Form W-4 to make sure that my tax withholdings were correct so as not to get a big bill for underpaid taxes.