Not necessarily. If I understand your question correctly, two cell LiPo packs are commonplace and can be used on a system with a BEC. Just check the ESC/BEC to see what the voltage range is. If it includes the 7.4 volt 2 cell pack, then you're all set. Just about any brushless motor will operate at 7.4 volts, check the KV rating.. that's RPM per Volt rating.. and see the difference between operating with a 7.4 or 11.1 pack.

This is an example of a typical 10 amp (15 amp surge) ESC with BEC. Note the info that says 2 - 4 cell LiPo packs..

http://www3.towerhobbies.com/cgi-bin...?&I=LXGVR9&P=7

CGr.