  Van der Waals (vdW) heterostructures consisting of bilayer graphene (BLG)
encapsulated within monolayers of strong spin-orbit semiconductor WS$_2$ or
ferromagnetic semiconductor Cr$_2$Ge$_2$Te$_6$ (CGT), are investigated. By
performing realistic first-principles calculations we capture the essential BLG
band structure features, including layer- and sublattice-resolved proximity
spin-orbit or exchange couplings. For different relative twist angles (0 or
60$^{\circ}$) of the WS$_2$ layers, and the magnetizations (parallel or
antiparallel) of the CGT layers, with respect to BLG, the low energy bands are
found and characterized by a series of fit parameters of model Hamilatonians.
These effective models are then employed to investigate the tunability of the
relevant energy dispersions by a gate field. For WS$_2$/BLG/WS$_2$
encapsulation we find that twisting allows to turn off the spin splittings away
from the $K$ points, due to opposite proximity-induced valley-Zeeman couplings
in the two sheets of BLG. Close to the $K$ points the electron spins are
polarized out of the plane. This polarization can be flipped by applying a gate
field. As for magnetic CGT/BLG/CGT structures, we realize the recently proposed
spin-valve effect, whereby a gap opens for antiparallel magnetizations of the
CGT layers. Furthermore, we find that for the antiferromagnetic orientation the
electron states away from $K$ have vanishingly weak proximity exchange, while
the states close to $K$ remain spin polarized in the presence of an electric
field. The induced magnetization can be flipped by changing the gate field.
These findings should be useful for spin transport, spin filtering, and spin
relaxation anisotropy studies of BLG-based vdW heterostructures.
