  We present a detailed analysis of harmonic longitudinal and Hall voltage
measurements for in-plane magnetized Pt/Co/Ta and Ta/Co/Pt trilayers in
reference to Pt/Co and Ta/Co bilayers. Enhancement of spin-orbit torques (SOTs)
and unidirectional spin Hall magnetoresistance (USMR) is achieved by
introducing the second heavy metal (HM) with the opposite sign of the spin Hall
angle. The extracted SOT efficiencies are larger for the trilayers as compared
to the bilayers, confirming the enhanced values reported for the trilayers with
perpendicularly magnetized Co. The maximum effective spin Hall angle found for
the Pt/Co/Ta trilayer reaches $\theta_{SH}$ = 20%. The USMR of the trilayer
yields up to 27% higher effect as for the respective bilayers with the largest
effective USMR amplitude of -0.32 $\times$ 10$^{-5}$ for the Pt/Co/Ta trilayer
at a charge current density of 10$^{7}$ A/cm$^2$.
