I both agree and disagree with the previous answers.
I personally provide both tax and bookkeeping service without a problem, but I definitely understand the concerns. Not all accountants/bookkeepers are created equal. While some do have specialties, others offer a full set of services. The advantage to having one good accountant/bookkeeping doing both is that he/she is thoroughly aware of your business and its financial situation. That makes it easier to decide how your chart of accounts should be set up and perhaps even offer you tips on how to improve your record keeping, billing, POS, collection efforts, payroll processing, quarterly filings, etc. It also allows them to point out other tax savings they may not be aware of otherwise.
Where I mostly agree is the oversight comment. As a business owner, you must be knowledgeable of every aspect of the business's activities because you are ultimately responsible. Ignorance is not a defense in the eyes of the law, so saying that you had no idea something is wrong, or placing all the blame on this one person, will not prevent you from paying the penalties that come along with embezzlement, tax fraud, money laundering, violation of financial regulations, etc.
With that said, always sign a contract (retainer) with any 3rd party you utilize (non employees) and make sure they understand what is expected of them; especially what deliverables are due when (financial reports, invoices, payments, etc). That way, you have some legal protection, and a means to cancel the contract, should they fall behind in providing the services you are paying them for. As a non employee, it's the only way to provide some level of protection for both parties involved because it clearly establishes the business relationship, including the cost or pay rate (locking it in for 6 to 12 months), the total hours expectes to be worked per month, the circumstances under which either party can back out of the agreement, when additional costs are warrantied, etc.
Also, a retainer usually means that you are at the front of the line because your payments are guaranteed. And if tax filings are included, you should be saving a lot of money compared to paying for two people instead. Just be sure to go over the contract thuroughly, and have regular meetings where you can review the financial reports they provide and ensure that you understand what they mean, and can identify any areas of concern. That information is vital to the success and compliance of your business, and providing feedback and updates of your business will allow them to better serve you.
Again, while they are responsible for their own activities, you are ultimately responsible for the entire business, including their handling of your books/taxes.