Legally, you can use one account, but are advised not to: having a single account and comingling the funds can make it more difficult to prove tax deductions, and also mean that the assets of company A are vulnerable to a debt owed by or judgments against company B, since it is difficult to tell where one company's assets end and another one begins. Despite more paperwork and somewhat decreased flexibility, you will enhance your protecdtion from liability and your ability to take business expense tax deductions from separating the accounts.