new H1B and L1B extension at the same time


My L1B with employer A is expiring in Nov 2012 and another employer B has applied for fresh H1B with COS, it is still under process.

My question is, can I apply for L1B extension with employer A and then leave employer A once I get my H1B with employer B.

What happens if employer A forgets to cancel the L1B extension petition and it also gets approved by some chance.Then will I lose my H1B based on last action rule by USCIS?.