I am working with company A on L1B visa (Visa already expired but I94 is valid until 2015 and company's blanket I797 petition valid indefinitely)
Company B filed for my H1B (with COS) visa which got approved on Sep 04, 2013. Now, I want to continue with company A on L1B visa.
Note: I cannot go out of Country as my L1B visa is expired.

I have already asked company B to withdraw my approved H1b application as I cannot join them due to some personal reasons and have kept a record of that email - haven't heard back from them yet.

Is that sufficient for me to keep working on L1B visa with company A i.e. without any acknowledgement or confirmation of withdrawal from company B or USCIS?

Thanks in advance!

You can't work on any expired work VISA... if its expired... its NOT valid until any future date.

Your status here would be illegal and will complicate any future applications.

The I-94 fact sheet link.

http://cbp.gov/linkhandler/cgov/newsroom/fact_sheets/travel/i94_factsheet.ctt/i94_factsheet.pdf

You need to deal with the expired VISA first.