If you are going to connect to BES, then you need the BES data plan. You should be able to drop your other data plan as BES includes Internet and other RIM related services.
However, many of use here do not recommend putting your personal BB on the company BES as the IT Policy can really lock it down and limit what you can do. Plus, make sure that you read this: What can a BES Administrator xxx8220;seexxx8221; on my BB... What does BES see; what can be tracked/logged? - Port3101.org : Your BES Connection