How can I find the location of another iphone on the same account?

The only way that you can track an iPhone is via Find My iPhone, or in iOS 13, Find My, then select Devices. When you say, "on the same account", I'm guessing you are referring to the same cellular account, your provider. That has nothing to do with it. You can only track a device if you have the Apple ID and password of that device. If not the same as yours, you would use a browser on the computer and log into www.icloud.com#find