So I was playing around with this widget and I noticed that when the spring masses are the same, the energy is completely exchanged (note one has to set the "show graph" option to "energy" and increase the time a bit). However, when they are not the same, the energy is only partially exchanged, although the oscillators are at the same frequency! (I decreased $m_2$ and $D_2$ by a factor of 5). Why is this the case? Do I just need to solve the differential equations or is there a simple explanation?