  Hole spin qubits are rapidly emerging as the workhorse of semiconducting
quantum processors because of their large spin-orbit interaction, enabling fast
all-electric operations at low power. However, spin-orbit interaction also
causes non-uniformities in devices, resulting in locally varying qubit energies
and site-dependent anisotropies. While these anisotropies can be used to drive
single-spins, if not properly harnessed, they can hinder the path toward
large-scale quantum processors. Here, we report on a qubit in planar germanium,
where we control the anisotropy of two spins in a double quantum dot. By
characterising microwave-driven singlet-triplet qubits, we show two distinct
operating regimes depending on the magnetic field direction. For in-plane
fields, the two spins are largely anisotropic, enabling to measure all the
available transitions; coherence times exceeding 3 $\mu$s are extracted. For
out-of-plane fields, they have an isotropic response but preserve the
substantial energy difference required to address the singlet-triplet qubit.
Even in this field direction, where the qubit lifetime is strongly affected by
nuclear spins, we find 400 ns coherence times. Our work adds a valuable tool to
investigate and harness the anisotropy of spin qubits and can be implemented in
any large-scale NxN device, facilitating the path towards scalable quantum
processors.
