Can a iPhone 3G have 2 lines on the same phone?

Yes. But you need to ask your provider to load your two lines onto one SIM card. This depends on the operator. i.e., not all operators provide such SIM cards.

If they do, under settings/phone you will have a section called "SIM applications", where you can switch from one line to other.

Two lines cannot be active at the same time. But you can forward incoming calls of one line to the other. In this case incoming calls coming to both of your lines will reach you, but you will only be able to make calls from your "active" line.