To have /tmp and /home on the same partition as each other but a different partition from /, you'll need to do this:

1. Make tmp and home the two top-level directories on the partition you want to contain them. So if right now that partition is itself /tmp, you'll need to move its entire contents to a tmp directory in its root.

   
   



2. Give the partition that contains tmp and home its own mount point. A reasonable name for this is tmp+home (unless you plan to use that partition for other stuff too) and reasonable locations for it are /, /mnt, and /media. I recommend putting it directly in / and the rest of this answer assumes that's where it is.

   
   



3. Give /tmp+home its own entry in /etc/fstab. (You will see /tmp has its own entry.)

   
   



4. Make /tmp and /home symbolic links to /tmp+home/tmp and /tmp+home/home.

   
   

Please note that this will not involve using these instructions for creating a separate /home partition, because in this configuration, /home will be a symbolic link into a separate partition, rather than a mount point for a separate partition.

Furthermore, you'll have to unconfigure your separate /tmp directory (including removing or, better, commenting out its entry in /etc/fstab), since /tmp will no longer be a mount point for a separate partition, but will instead be a symbolic link into a separate partition.

This answer doesn't provide extremely specific instructions (though it should be sufficient, combined with the available documentation). If someone who has experience moving /tmp can weigh in on whether or not it's possible/safe to move /tmp while in the installed system, that would be helpful. I can give detailed instructions for doing this from the installed system or from a live CD, but I don't want to give directions that are ineffective or unsafe (nor directions that are unnecessarily difficult and time-consuming).