How can I find my other iPhone

If you used the same Apple ID then Find My app will show both phones.

Unless of course the other phone is now turned off.

phone stolen or lost

Learn more about how use the Find My App.

Find My - Official Apple Support

Then look at this Apple article and my additional advice below it:

If your iPhone, iPad or iPod touch is lost or stolen – Apple Support (UK)

Tracking lost or stolen phones

The 'Find My' tracking is to help you discover if you left your phone in the office or the restaurant. Lost or Stolen phones are rarely ever seen again. Unless you have your own armed gang you will not want to contact the organised gang that now have your phone. 

Risk mitigation

Get your provider to 

Cancel your SIM.

Send you a replacement SIM on same number.

You can then put the SIM in another iPhone to get the codes you will need.

Phishing warning

Its possible for a thief to obtain your phone number particularly if you did not have a PIN set on the SIM.  So do not respond to texts to sign into your account! They are NOT from Apple. 

Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

Resign to the fact that if you had a passcode set and you do not provide your Apple ID via phishing texts then your data is safe but you may never see the phone again.