I want to do Dumbbell Gain + Ultimate Burn in the same period.
They are two complementary workouts: one focuses on muscle mass, the other helps burn fat.
I can do one in the morning and the other one in the afternoon, why not?
The same reason why you have to commit to one goal & one trainings plan in general.
There are also Journeys, which do both. For Example Freeletics Hardcore has 3 Dots Cardio and 3 Dots Muscle. You can also always tell the Coach in one Journey, that you want a “harder” session and it will adapt.