Without scripts, you need at least 3 skills and one Common Event. One skill (50-50) only calls a Common Event as a feature and your other two is PDF 100% and MDF 100%.
Your Common Event should be something like
if script: rand(2) == 0
-Force Action: PDF 100% - Caster on Last Target
else
-Force Action: MDF 100% - Caster on Last Target
end.
In theory, casting 50-50 will run this common event, which will then cast one of the two skills depending on what rand(2) generates (0 or 1). Last Target should take the last target of the caster, and since we targeted this through 50-50, the MDF/PDF would be cast on that same target.