Why can't I make two modifications on the same file and check in them in different changeset ?
Accepted answer
You can do that if you "Complete" the first change set. This means you can no longer modify that change set, but you can now check in additional changes to that file as part of a different change set. See http://pic.dhe.ibm.com/infocenter/clmhelp/v4r0m3/topic/com.ibm.team.scm.doc/topics/t_marking_change_complete.html