  We identify an optical excitation mechanism that transfers a valley
polarization from photo-excited electron-hole pairs to free charge carriers in
twisted WSe$_2$/MoSe$_2$ heterobilayers. For small twist angles, the valley
lifetimes of the charge carriers are surprisingly short, despite the occurrence
of interlayer excitons with their presumably long recombination and
polarization lifetimes. For large twist angles, we measure an increase in both
the valley polarization and its respective lifetime by more than two orders of
magnitude. Interestingly, in such heterobilayers we observe an interlayer
transfer of valley polarization from the WSe$_2$ layer into the MoSe$_2$ layer.
This mechanism enables the creation of a photo-induced valley polarization of
free charge carriers in MoSe$_2$, which amplitude scales with the gate-induced
charge carrier density. This is in contrast to monolayer MoSe$_2$, where such a
gate-tunable valley polarization cannot be achieved. By combining time-resolved
Kerr rotation, photoluminesence and angle-resolved photoemission spectroscopy
measurements with first principles calculations, we show that these findings
can be explained by twist angle dependent interlayer scattering mechanisms
involving the Q- and $\Gamma$-valleys.
