Why???? /u01 and /u03 are not memory, they are disk volumes and it will cause NO degradation of performance to have the data files on different spindles unless /u03 are slower disks then /u01. Balancing of disk usage between two drives will make no difference.
You do not need a parachute to skydive. You only need a parachute to skydive twice.
No downtime required at all, just create a new tablespace with two datafiles, one each partition (/u01 and /u02) and start moving segments from the tablespace with datafile x-file1 to this new tablespace.