I don't think that's true. You can only observe that inertial mass is not equal to the gravitational mass if you have two objects of different material with dif

I don't think that's true. You can only observe that inertial mass is not equal to the gravitational mass if you have two objects of different material with different values for inertial mass/gravitational mass. If the big and the two half sized objects are made of the same material then a different inertial mass just corresponds to a different gravitational constant G. Mathematically let m_g be the gravitational mass and m_i be the inertial mass. We have F = m_i*a and F = G*m_g*M_g/r^2. So we have a = G*m_g/m_i*M_g/r^2. Now m_g/m_i is the same for the big object and for the half object. We can absorb it into the constant G and end up with a = G'*M_g/r^2. Conclusion: the big object and the half object have the same speed.