Sorry I think we are talking cross purposes..
What you have is a secondary virtual disk.. plus the TM virtual disk (its sparsebundle).. but none of your screenshots have shown the second partition .. only the time machine partition and I am not following the language in the screenshots..
What you have done is fine..
But you have NOT partitioned the disk.. Going back to the original post.. you told Neil you have partitioned the disk.
When I bought my TC I followed a tutorial in order to keep a portion (500GB) of my 2TB TC free from TM backup in order to store my temporary files.
I went to disk utility and used the Partition TAB. Now it looks like my TC has 2 partitions, as it is explained when I click on the big or the small one...
Is there something wrong in this?
Ok.. so you understand this.. You did not partition the TC at all.
You created a sparsebundle or dmg file in the one partition.. it is a virtual disk.. not a partition.
It functions as a partition and from your point of view works fine.. but it is not the same as creating physical partitions on the disk.. so if you go to airport utility and erase the partition you will erase TM sparsebundle and your "partition" and everything.. in a true partition scheme you need to erase each partition separately.
I hope that all makes sense now..
And sorry I did misunderstand what you posted.