Can I partition external HD to back-up win7 and SL?

I have been trying to partition my external 320 Gb iomega ego helium into a 200/120 partition to back up SL via time machine and win7 via windows backup. Each attempt gives me 2 partitions of 160 Gb each. Am I attempting the impossible? Yes I am using the SL disk utility to attempt the partition as I am uncomfortable allowing Win7 any such tasks.

When you are in Disk Utility and attempting to partition, that is where you can adjust the size of the 2 partitions. You can certainly make one 200GB and one 120GB. Just be sure the Win7 partition is NTFS and the Mac partition is Mac OS Extended (Journaled) if you intend to use Time Machine for your Mac's backups.

Drag that "bar" up or down to change the size of the two partitions. Or manually enter the GB size in that box.

Thanks for the info. I did not realize that ExFat and NTFS were the same thing. Backing up now.