The only real way you can get any kind of half decent regulation in those 2 outputs , is if they are always only ever both fully loaded on constant load all the time.
You should try and arrange the turns so that the duty cycle woudl be the same for each when on the full load.
There are all kinds of tricks to "improve" cross regulation in any SMPS....pretty much none of them really work.
There is one method using active inductors, to allow diode conduction or not.
Then theres bridge converters with coupled output inductors...but its all pretty rotten.
You can also try multiple interleave winding.......but it takes much room on the bobbin.