I have the same number on 2 different phones with 2 different carriers I recently switched carriers and everything was done over the phone. I provided my account number etc from my old carrier to my new carrier and authorized everything to be transferred over. 

I purchased a new sim card for my new carrier and placed it in my iPhone 6. When I called back to activate the iPhone 6 I called from my old iPhone 4, which was registered with my old carrier. 

I expected as soon as the iPhone 6 was activated that the iPhone 4 with my old carrier would cut out, but it doesn't seem like this was the case. Now I have both phones connected to separate networks each using the same number and are fully functional. 

I didn't think this was possible, does anyone have an explanation?
When porting a number from one provider to another, it may take a few hours for the old network/provider to complete the process. If the situation still exists after a full business day, give the old provider a call to sort it out.