From what you're saying you can't be in status on both visas at the same time. Well, you could be, but it would be onerous. I understand that you were present on the first visa, left, and reentered on the second visa, or never entered on the first. That makes sense. But from what I understand of your situation you can't remain in status (or also be in status) on the second visa if you're working only on the first, and I doubt you intend to hold down two full-time jobs. Thus, my reaction is that you should transfer your second visa, or go to a port of entry and reenter on your first visa so that your I-94 reflects the employer you intend to work for (take with you your approval notice for the first employer). There could be portability issues with the first H1-B that would not be present with the second.

