At the time of I-485 filing you must have intent to work for the petitioning employer. You cannot intend to work for both I-485 sponsors, so BCIS will ask:
"Were you lying on your first adjustment application, or your second?"
---Many employers can sponsor one for future Employment; employer shows the intent to hire when he files I-140
--- Employee files I-485 based on that I-485 and employee could change intent any time after filing I-485
--- The person is not lying on second I-485 many have other issues like if the previous I-140 is approved and I-485 is pending the person may be still in confusion if he can use AC21 as recent BCIS Memo clarified few points related to I-180 days but what is same/similar job is still not defined.
--- The person can file second I-485 both the I-485 will not be having PD of same date, what ever is approved first one accepts that and once GC is stamped on PP then all other pending I-485 or I-140 with CP option automatically become void.
I know a case the person had I-140 approved from VSC and filed I-485, he then left the company A who filed I-140 and his I-485 was based on I-140 of A (he was not clear if he can use AC21 or his employer may revoke I-140)
He then joined company B by H1 transfer got new LC approved, filed I-140 and I-485 concurrently with NSC as employer H.Q is in NSC. The lawyer who filed it is reputed lawyer, the lawyer told the client that once I-140 of company B is approved that can also be attached with pending I-485 with VSC and request can be made that please approve this I-485 on based on I-140 approved by B
Nothing is illegal in filing second I-485, one has reasons to file it and employee is ready to spend on medicals and pays the lawyer fees, if he gets peace of mind he can file second I-485 and it is not illegal