What phones do you have? All phones have access to Verizon's backup assistant, and each phone has it's own account. You can access each line's account by going to verizonwireless.com and registering each number for an account, with separate username and password for each line.
If you have a smartphone, there are backup apps, and you can use Google, or Windows Live, or iTunes for backup depending on the phone OS. In all cases, each phone has it's own backup.
