Lets be serious here.
1. Any pilot that can fly either a 380 or a 320 well can fly both well at the same time. The TEM skills required to fly different types are the same ones required to fly one type.
2. Many pilots struggle to fly one aircraft well.
3. A pilot who struggles to fly one aircraft well will have a more difficult time flying two different types at the same time.
It all comes down to training. My most recent sim partner had the SOP down so well that every single checklist response was letter perfect. Unfortunately this meant that every time we did landings without the autothrust the checklist response for "Autothrust" was "Speed", despite the FMA being blank. Two different instructors saw this and said nothing. If a training department can't teach the basics, it's unlikely that the pilots will fly well, MFF or not. We used to change aircraft type every six months and after each switch the incidents went through the roof (including multiple aircraft climbing through 10 000' without pressurization). The solution decided upon by management was a stern lecture to the struggling pilots in question and an in increase in the quantity of poor training.