Making bootable USB with Lubuntu

I want to make a bootable USB with Lubuntu on a USB with 16 GB. I want to access files on it when I am not using it. I am wondering if it works to partition it to two 8 gb USBs and put Lubuntu on one partition and use the other partition to access my files when I am not using Lubuntu.
Questions and Points.
- I am making a bootable USB because the computer I want to use it on is really slow, and I want to use a faster operating system.
-Can I access files on the hard drive of the of the computer that I use the bootable USB on? I have songs on it, and I might want to use it as a local data storage.
-In Universal USB Installer, what is the setting for that says "Set a persistent file size for storing changes."? Is it the same thing as making a partition for the Lubuntu OS?