H1B petition and notice period

[basak]

My situation is, I am currently on H1-B with employer A, got an offer from employer B and new H1B petition is filed by employer B with USCSI. Waiting for approval on new petition filed by Employer B. I started working with employer B even without resigning from employer A. Right now I am working with both Employer A and Employer B. I am waiting for approval from USCIS for the petition filed by Employer B. Also a point to note here is I have applied in premium and from July 12th the status is showing as "USCIS is actively reviewing the application". As soon as I get approval I will resign from Employer A, serve notice period and start fully working with Employer B. Is it legal to work like this, with both employers for brief period. Is it legally acceptable as both of the employees will Payroll and will this create any issues in future if I want to apply for PERM and I-140 in future.