A handset does not require 2 antennas to monitor more than one cell, the handset maintains a list of the monitored cells in something known as the active set along with their power level, I am unsure if this information will be available through APIs to a programmer though.

Only devices using MIMO will have more than one Cellular antenna as mentioned before, however I believe that UMTS (3G) also supports MIMO but only for HSPA+ at speeds not currently implemented i.e 21MBps+