There's no compliance issues when it comes to all time as they're gonna make sure they take their drugs or somebody will because they can't remember anyway
It may also be that each patient needs a varying amount of each. Could make more sense to have them separate. Also, a healthy person trying to prevent AD may just want 2-73. Honestly I think the market is MUCH bigger for 2-73 alone. I believe it's clear why they work so well together when AD is already present. I have a feeling the combo may slowly phase Don out as the brain becomes healthier it may only need 2-73