How do I install Ubuntu or a derivative over two mediums?

what do you mean when you say "medium"?
what exactly are you trying to do?

By medium, I mean physical places I can install to, like a hard drive, SSD, flash drive, etc.

it still isn't clear what you're trying to do?

I am trying to install Ubuntu (or a derivative) on a filesystem that is on multiple drives

You can install one Ubuntu instance by disk partition, but as Marcus, I need to understand what you want to do with this. They could be more cleaner and/or simpler solution. E.g. people use two instances: Ubuntu and Kubuntu to have two graphical environment, but it's cleaner to have one instance and choose at login which one to use. You can also have problems if you share "/home" folder between two OSes.

Sorry, you posted before me.
If you have a file system on multiple drive, I deduce you use LVM (Logical Volume Management) to build a virtual disk on which you make partitions. Could you confirm ?

No, since all of Ubuntu will not fit on a 2GB SSD, I am wondering if I can install most of it on that 2GB SSD and the last small part on a flash drive or something

How did you install Fedora on two drives ?
At installation, Ubuntu Lucid 10.04.2 uses 2.3GB plus swap.
SSD disks are better used in reading. The folder "/usr" uses 1.7GB, which will give you some margin if you install software. You could make a manual partitioning with:
SSD
   ext4fs /usr
flash driver
   ext4fs /
   swap