Is it possible to add the Behavior Score and Demographic Score

Not that I'm aware of.  See this idea:
https://community.marketo.com/MarketoIdeaDetail?id=08750000000IGLbAAO

The easiest way is to increment them equally at the same time. For example, if you have a lead scoring trigger that increments behavioral score, increment total lead score by the same amount. As long as you keep all your scoring triggers in check, this will achieve what you want. More work, yes, but still gets you there.

We do this via a formula field in SFDC. Since our scores feed back into SFDC, we have fields for each of the three, then a calculation that figures out the overall score and thus MQL threshold. Based on that, and when it changes, we can assign the MQL.

I noticed this morning that you can create new fields of the type "Formula" now. I'm not sure how long this has been available and I haven't tried or tested it out yet, but I am going to because we have the same issue here with demographic + behavior scoring. We're currently doing what Drew recommends by incrementing both values at the same time. 

Updated:

Just tested it out and it doesn't work how I thought it would and apparently that field type has been available for some time. Oh well. We'll stick to how we have it set up I guess!

The Formula field is a string, not a number or score, so it won't work in this situation.