you should be able to resize the NTFS partition using gnu gparted or gpart or nparted tools. look for features before you do any changes and do it at your own risk. once you resize the nfts partition and shrink it you can create a new FAT or FAT32 partition with that free space. once you have that FAT/32 partition you can write onto that partition from linux or mswin. the files that you uses between both the os should be kept there. hope this helps

just a note:
i have never used the above mentioned partition tools but have heard good things about it.
recently i used partition magic 8 to do exactly similar thing. f**ked up my hdd and gives lot of errors in mswin. i know in and out of hdd geomety and know its f**ked up and the only way i can fix is physically destorying it. i just tried shrinking partition to add a fat32 partition. the goal was not to put linux but something else.
so just beaware.