If you are in an area of poor coverage, be it 2G or 3G you will enter compressed mode. Where you will monitor both 2G and 3G cells, effectively having both transmitters/Receivers active.

There are settings in the Mobile network that control, when compressed mode is entered and how many times with in a period and how many times it is allowed to switch between 2G and 3G within a period. This is as far as I am aware uncontrollable from the Handset.