The accounting issues are the customer's problem, but you don't want to be dragged into any investigation if you can help it.
The customer can make transfers. Using a check and a deposit slip accomplishes the transfer AND provides the required record to be retained under BSA.
Bill pay would also make the transfer and your bill pay system should maintain the required funds transfer records. You may also have the recordkeeping requirements built into your telephone transfer procedures.
Online banking with single access to both accounts and the ability to tranfer back and forth between accounts with different ownership may not have the required recordkeeping covered. The ownership of each of the LLCs can change at any time.
Al
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Al Miller, CRCM
Opinions expressed are my own and not necessarily shared by my employer.