Can you use a bootable macOS USB created on a MacBook pro, to install macOS on another MacBook Pro?
Yes, it is perfectly normal to create a macOS bootable installer USB key on one Mac and use it to install macOS on any other Mac.

The bootable installer is not customized for a particular Mac hardware, can be used to install macOS on any Mac that supports running the particular version of macOS.

You can use your 2009 MacBook Pro to download OS X El Capitan installer from this link:


OS X El Capitan
How to upgrade to OS X El Capitan


After downloading the installer, you can follow the below instruction to create to create a bootable USB installer:


How to create a bootable installer for macOS