When thinking about drogues, think about terminal velocity. If you separate the rocket into various pieces (fin can, nose section) each will reach their own, particular terminal velocity when the drag force equals the weight of that section. You don't even have to compute the actual terminal velocity, just a relative terminal velocity.
Measure the cross sectional area of each part of the rocket, as if it were falling sideways. Take that number, divide by the weight of that piece. Bigger numbers will have a lower terminal velocity.
When you tether them together, the part with the larger terminal velocity will make the slower piece speed up, which means it will fall seem to 'hang' below the slower part. If the difference is significant, then you don't need a drogue to keep the pieces seperate.
When you atttach a droge to either piece, it's like adding lots of area, and very little weight. You can simply add in the area of the chute and the weight of the chute and re-figure the relative terminal velocities.
This will give you a good idea of the configuration in which the rocket will fall. it woun't give you a descent rate for the whole disassmbled rocket.