I lost my second iPhone how can I locate it by using my other iPhone

You can use Find My on another Apple device (can be an iPhone, iPad, or iPod Touch) or through iCloud.com. Usually it's turned on by default but there's a possibility that it was turned off for one reason or another and not turned back on.

Find My - Official Apple Support

Use the Find My app to locate a missing device or item - Apple Support

iCloud - Find My - Apple

Locate a device in Find My iPhone on iCloud.com - Apple Support

If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

If Find My was turned on in the same Apple ID use the Find My App on your iPhone.

Login with your Apple ID using Find My app or

use a browser => https://www.icloud.com/#find

provided you have Find My iPhone ON before lost.