Two Networks?

As in AT&T and Verizon at the same time? No.

What were you trying to do?

If you really need two networks at the same time you could always use a hotspot/MiFi from the other network...

B

Technically, your iPhone is connected to two different networks. 1x for voice and EVDO/LTE for data. (if you're on VZW, Sprint)

dual SIM phones

In China and South Asia I have seen many cell phones with dual sim cards. I bought a cheap GSM phone during my travels and one can run a phone on 2 or 1 network at the same time.