H1B extension plus transfer in parallel

Hi

I am currently employed with a company A which is filing for my H1B extension. MY I-94 and H1B petition is valid till 12/31/2011.
I have got an offer from a company B which is going to file for an H1B transfer.
I want to understand if both an H1B transfer and extension can be done in parallel.

From a few forums that I have read, it seems that one should wait for an H1B extension to be approved before filing for a transfer.
Does anyone know if there is any rule which disallows H1B extension and transfer in parallel?

Also, if both can be done in parallel which petition would take precedence?
I would really appreciate some advice on this
Thanks in advance