It probes both network simultaneously - that's also why You can have both data connection with high bandwidth usage and non-interrupted calls. Generally, it is't overkill - becomes grand power sucker while traveling through regions with changing signal strength (both 2G and 3G is probed intensively), or when one (or both, obviously) range is barely reachable (low 2G or low 3G signal).

As a rule of thumb, it's better to sit on 2G, and enable 3G (even in dual) when You need data connection. In some special cases, it's also good to use 3G only for calls - they're much better encrypted, as 2G was broken long time ago (yet, it's not trivial, so You can rather don't expect script-kiddies eavesdropping conversation, even on 2G). Overall, eavesdropping 3G is *much* harder, and require *very* specialist equipment, and/or cooperation from Provider, where 2G theoretically can be decrypted by tech-savvy hobbyist, with some special (still expensive, if hobbyist in question isn't tech guru and haven't DIY it) equipment.

/Estel