Provided the both objects are identical except for mass, they will fall at the same rate. Ignoring drag from the air for a moment, eg in a vacuum, gravity acts to accelerate each particle at the same time.
Consider this thought experiment: Three identical masses, dropped at exactly the same moment. Each will accelerate the same as the other two. Now connect two of the masses by a loose piece of string that is so small that its mass & surface area are negligible (or even non-existent if you posit a thread small enough). Do you think the two connected masses will suddenly behave differently to the independent mass? Now imagine the thread replaced by a rod so now the two masses are locked together. Do you think the connected masses will suddenly change behaviour? If it helps, imagine the connector suddenly appears mid-fall. Now imagine, instead of three identical masses with two of them connected together, two identical masses differing only in density. Gravity's effect is the same, regardless of size or shape.
Now add a fluid such as air. The only effect the fluid has is to provide a retarding force - 'drag' - reducing the acceleration of gravity. Drag is not affected by mass. C=Cd 1/2 rho v^2 s remember. So, as long as the objects *only differ in density* they will fall at the same rate.
Last edited by Tinstaafl; 12th March 2014 at 11:30.