Is it possible to have two phones (one of which an iPhone) on the same number without changing the SIM card?

I have always wondered whether it was possible to use the same phone number on an "emergency phone" in case the main one gets lost.

Yes

Yes it is possible. I think they developed this for the fixed car-phones. Some providers support 2 simcards with the same number. How it works:

• The phone that had a connection and some communication to the cell-tower last will be the primary phone.
• When you make a call, that phone becomes the primary phone.
• When you change locations (change cell-tower), that will be your primary phone.
• If your primary phone is not on the network, your secondary phone will become the primary phone.

The primary phone is the only phone that can receive calls. Both phones can call. Ask your provider for more info.

This is based on my experience (Vodafone, Netherlands). And I am sure there are better descriptions out there ;)