NYC Tax Lot - Combining 2 Apartments

[Kevin Seith]

Hi, I recently bought the neighboring apartment (condo) next to the unit I currently own in Manhattan and I am looking to combine the two units. When I submitted the construction plans to the DoB I was also required to submit a form for a "tentative" tax lot number. My question is, am I required to take the next step and file with the Department of Finance to formally combine the tax lots and have 1 tax bill? Or would it be cheaper, from a tax assessment perspective, if I kept them separate? Thank you in advance for all your help!