Well, the
best thing to do is repartition to have a place to put the backups in isolation. You can partition "live" with iPartition from Coriolis Systems.
But, there are alternatives. To just copy to the source, without erasing anything, you can use either "Copy Newer" or "Copy Different", neither of which will erase files (though some might be overwritten). Note, though, that this type of backup is NOT guaranteed to be bootable, since OS files might need to be erased to ensure this will work.
Alternatively, you can back up to a sparse image as described in
http://www.shirt-pocket.com/forums/showthread.php?t=81. That can stay side by side. While an image can't be booted from directly, if updated with Smart Update it'll retain its bootability on restore.