Motorola began trading its shares as two separate companies on the New York Stock Exchange (NYSE) Friday.

Motorola Mobility, its handset arm, is trading under the ticker symbol MMI WI; Motorola Solutions is trading under MSI WI. The stock is being traded on a “when issued” basis, which helps determine the opening price for new stocks.

The company is still trading as a single company under MOT and will continue to do so until Jan. 4, 2011.

Shares of Motorola’s handset arm fell more than 11 percent from Friday’s high of $27.50 and were trading at about $24.50 as of 9:40 a.m. Central today. Motorola Solutions is trading at about $40.

Motorola Mobility is under pressure to maintain its profitability as a stand-alone company. The division posted regular losses until the third quarter, when it posted non-GAAP (generally accepted accounting principles) profits of $3 million but still lost $43 million on a GAAP basis, which includes expenses for stock-based compensation, intangible amortization and reorganization charges.