Cannot be done.
The difference between 3G and 2G with regard to battery usage is realy not that big.
What kills the battery the most is switching between different networks.
It is a pitty you cannot chouse to lock on 2g network only. I was a bit dissapointed too.
I hope setting such as these do get enabled in the future updates.
Well you learn something new every day.
My phone does not have that option.
I'm sorry for missinforming...
Do both of your phones have the option? HD7 and Lumia 710?