Multiple H1Bs but entering on a former employer's visa. Does that allow me to stay in US per that I94?

I had multiple H1-B (797s)at the time I went to India , one with company A and another with company B. I had received an offer from company B but hadn’t joined B yet and joined B only after i came back from India. So i reentered US with company A’s visa and i got my I-94 till Sep 2015 (max date on company A’s visa). I have been asked to go to India to re-stamp with company B’s visa and reenter US by B’s attorneys. Is it considered illegal even though i have a valid visa in hand and I94 with the new company?

You mentioned I-94 until Sep 2015. So that’s already expired; or did you mean a different year?

Generally, you can appear for stamping even though you still have unexpired visa from a different employer. In my opinion, it is not required to get B’s visa stamp as long as A’s visa stamp is valid. You can leave US and return on A’s visa stamp and B’s petition to get I-94 in B’s name. The attorney’s suggestion is also valid and he is playing it really safe here. You may be eligible for dropbox if A’s visa stamp is still valid or hasn’t expired more than 12 months ago.