Yes, you can put as many files in as many folders as you want. Until the drive is full, of course. And it's your own responsibility that files from comp A don't overwrite files from comp B by putting them in different folders.
You can install a program on an external harddisk if the setup allows you. It's up to them to do it or not. But if it relies on things the install puts in the registry, you'll need to do the install on both comps. And things might go wrong if this drive gets different drive letters on both comps. So it's somewhat tricky.
I've got certain programs I can run from a USB-stick, and if that works it should work from a external hard disk also. All I can say is try it, but don't trust on it until you know it works.
i have two comps that i need to keep backed up. will all external hd allow me to do that? and do i see that i won't be able to keep programs on it. thanks for any help