Yeah, this has happened to clients of mine a few times - they always end up merging in the end because the powers that be realize they don't want to be paying for two separate systems. Often there's a period of time where they do operate like you said, just do a month end TB load or something to capture true financials in the New NetSuite, but everyone keeps operating in the old NS for a while. It can be a decent-sized effort to move you off your system, depending on how much customization you're used to in your current account, you may need to do testing/large amounts of data migration, etc, but the cost savings of only 1 netsuite instance is usually worth it in the end.