Here's what you do:
in SFDC: create Behavior Score, Demographic Score as integer fields
wait
In Marketo
Go to those fields in Admin > Field Management and mark them as Score fields. They will now show up in the Change Score flow.
Now in the Scoring flow you'll do
Behavior Score +5
Lead Score +5
In a title score:
Demographic Score +16
Lead Score +16
So you don't add them together, but change them by the same amt each time. To reset demo scores and lead score, do
Change Data Value: Lead Score = {{lead.Behavior Score}}
Change Score: Demographic Score =0