If you in Nautilus turn on show hidden files/folders you will see lots of the configurations, but most do not take much space. Some applications like Firefox & Thunderbird store all there data in the . or hidden folders.

I keep /home inside / (root) and link all my data back into /. I use 25B for most / partitions, but actually use about 9GB. My /home is 2GB (of the 9GB) and 75% of that is .wine as I install the Windows version of Picasa and wine is more difficult to move.

Having two different users and setting permissions for each and shared makes it a bit more complicated & I do not know details.

Splitting home directory discussion and details:
Link is on move home but see post by bodhi.zazen on data partition #6

I jsut use links, but some prefer bind or bindfs.
HowTo: Create shared directory for local users (with bindfs). - sisco311

Advantages of bind post 16 - Morbius1

I have two data partitions. One still is NTFS from when I dual booted XP and I have not moved that into my Linux partition, yet. I have my Firefox & Thunderbird profiles in the NTFS so I could have the same data in XP & all my Ubuntu installs. I also move some other hidden folders with data to my LInux formated data partition.

You will probably need to plan partitions or folders ownership & permissions, so you can separate your accounts and share some.

Some posts from those that really understand ownership & permissions with multiple users.
Share folder among two users of same PC.- morbius1
Two users to share music folder - group & permissions -BoneKracker:
All users read/write shared folder - morbius1