 Solution

Let the two layers have equal magnetic moments $m$ and coercive-field magnitudes $H_{c1}<H_{c2}$. Assume they are exchange-decoupled, both easy axes are perpendicular, each switches sharply, and dipolar coupling, thermal activation and domain distributions do not smear or offset reversal.

Starting from positive saturation and reducing the perpendicular field, the total moment changes asat $H=-H_{c1}$ and $-H_{c2}$. Between the switches, the moments are antiparallel and cancel. On the return sweep the corresponding transitions occur at $+H_{c1}$ and $+H_{c2}$. The plotted normalized loop therefore has two reversal steps and an antiparallel plateau on each sweep, as in the accompanying figure.

Equal magnetization densities do not alone guarantee an exactly zero plateau if the two Co thicknesses or active areas differ: cancellation requires equal moments $M_{s1}{V}_1=M_{s2}{V}_2$. The source allows Co thickness variation, so this is an additional assumption worth stating. A real unpatterned millimetre-scale film may reverse by domain processes rather than coherent rotation, producing rounded steps and reduced remanence.