Actually, to both Retail Brokerage accounts and to MDV's (market data vendors), just simply being listed with the SEC by means of your SSN (in the Brokerage databases) will force you to be classified as Professional.
That classification will also subject you to higher Exchange Fees (for market data feeds) and Regulatory Supervision over the account (namely duplicate confirmations of trades being sent to the licensee's compliance officer; preapproval of purchases on restricted stocks (potential insider implications); knowing what's restricted from personal trading; etc.).
There's a whole section in the Supervisory Manuals on watching Brokers conduct (read trading and personal accounts) once they become licensed. This supervisory oversight also applies to all employees and family members accounts, of employees working for Brokerage and Brokerage related firms.
Esignal used to directly check with the SEC, based on the application that one signs, as well as other market data vendors (like CPQ, PCQuote, and others). Most exchanges which derive revenue from selling their quotes and news, also require that packagers (bundlers of their data) differentiate from "regular customers" and "professionals (licensed individuals)" in their pricing structure.
So, yes, you can have an outside account, however, as long as you have an active license, please prevent the problems you'll be in, by first going to your compliance department / officer and posing the question to them. Remember, in these days of increases scrutiny its far better to be correct and accurate than ignorant (which is no excuse before the law).
Cheers ...