See clip from Attorney Rajiv S. Khanna's conference call video that addresses this question.
Answer to Q1. In other words you have the intention to join both of them but you haven’t chosen yet and I think that’s quite OK as long as that intention remains and the employer’s intention remains to hire you at the present date or future date you can file a green card or even an I- 485 that’s perfectly permissible. Actually you don’t have to join them on EAD you can join them after the green card is approved as long as you have the intention to join them the green card can go forward.
Answer to Q2. No you do not.
Answer to Q 3. The answer is yes
Answer to Q 4. The problem is USCIS can always question this arrangement from a common sense perspective. This type of situation has not been questioned very frequently. In my entire twenty plus years of practice of immigration law I have had only one instance out of Chicago where this issue was raised and questioned. We get a kick back from USCIS saying well you know if he wants to work permanently for this employer why is he working for the other one why did he leave and we had a very good answer. In fact being paranoid we had prepared for that and made sure we knew what the reason was and it was genuine. This company was specialized in JD Edwards ERP and JD Edwards had lost a lot of market share so the individual in question the green card employee was a senior technical person and he went to get experience in other types of ERP. He looked at SAP and he was doing work in oracle finances and applications. He was learning all these technologies and the idea was he would be able to bring it back to the employer and they would be able to expand their ERP offerings and we were able to get that green card approved without any problem. So even though he was working for another employer the prior employer got his green card approved and once the green card was approved he joined. By the way under AC21 once the I-485 has been pending 180 days you can file an AC21 without joining the prior employer. The intent to join has to remain active in your own mind till 180 days or I-485 pendency has passed.
Answer to Q 5. I think that’s OK.
Answer to Q 6. No because the government has made it clear we just want one active I-140 we don’t care which employer it has been approved with.