I worked for Company A. I quit Company A and now working for Company B.

From Company A, both my labor and 140 were approved . My priority date is Nov-08.
I want to join back company A when my priority date becomes current.

company B wants to start my greencard processing now. I have below questions.

Can I use my priority date (Nov-2008 from Company A) with Compay B green card? If Yes, then when do I need to specify the date? At time labor? or at the time of 140? or even later?
If I apply greencard through Company B now, what happens to my company A green card? will it be remain valid and active? or gets canceled?

I don't want to mess up my company A green card application, but want to start greencard with Company B. Is it possible? or any other suggestions?