Great Pacific Garbage Patch Equilibrium Points
Theoretically, I agree that there should be one instable equilibrium point between the two stable equilibrium points when we have a 2D current field. However, there are some additional components of the problem:


Ocean currents are not 2D but 3D currents. Considering only the sea surface might lets field lines disappear. 
The wind has an additional forcing. This is not true for all items and the wind already affects the ocean currents. However, litter items which are partly above the sea surface experience an additional wind forcing (see e.g. Yoon et al., 2010)
Ocean currents are not time independent and equilibrium points may undergo a seasonal multi-annual cycling
Accumulation zones are not necessarily stable equilibrium points but the residence time of the marine litter items in the named regions just needs to be sufficient long.
The gyres are macroscopic accumulation regions. Thus, if we have regions with a higher density of particle trajectories and lower particles velocities we could identify these regions as accumulation regions from a macroscopic perspective. 


About item three (Ocean currents are not time independent) I am not completely sure, but I think to remember an animation or plots in a publication which showed seasonal variations between the both garbage patches you showed.

As far as I remember the publications on that topic of global transport modeling of marine litter correctly, the eastern North Pacific Garbage patch is the actual accumulation region. It has a long tail towards west. Howell et al. (2012) writes about the North Pacific Gyre and plastic debris accumulation within it. I did not have time to read it in detail.

For further reading it might be interesting to look into these publications:


Eriksen et al., 2014. doi: 10.1371/journal.pone.0111913
Howell et al., 2012. doi: 10.1016/j.marpolbul.2011.04.034
Lebreton et al., 2012. doi: 10.1016/j.marpolbul.2011.10.027
Martinez et al., 2009. doi: 10.1016/j.marpolbul.2009.04.022
Maximenko et al., 2012. doi: 10.1016/j.marpolbul.2011.04.016
Yoon et al., 2010. doi: 10.1016/j.marpolbul.2009.09.033