Analyzing MF asset with assumable loan

[Scott Moore]

I have been using Michael Blank's SDA, and am wondering if anyone has experience with underwriting an assumable loan on his model? The assumable loan in question has a far lower interest rate than the additional debt needed on top of the remainder of the assumable loan. Would I just treat it like one big loan and use an average of the different rates? Would I create two different SDA's each reflecting the different rates, then add the information together? I assume there is something I am not understanding/seeing on the model. I know there is an option for a 'second mortgage' on his model but doesn't seem to be working for the numbers I'm looking at. 

Thanks in advance.