  In multiorbital materials, superconductivity can exhibit new exotic forms
that include several coupled condensates. In this context, quantum confinement
in two-dimensional superconducting oxide interfaces offers new degrees of
freedom to engineer the band structure and selectively control 3d-orbitals
occupancy by electrostatic doping. However, the presence of multiple
superconducting condensates in these systems has not yet been demonstrated.
Here, we use resonant microwave transport to extract the superfluid stiffness
of the (110)-oriented LaAlO3/SrTiO3 interface in the entire phase diagram. We
evidence a transition from single-band to two-band superconductivity driven by
electrostatic doping, which we relate to the filling of the different
3d-orbitals based on numerical simulations of the quantum well. Interestingly,
the superconducting transition temperature decreases while the second band is
populated, which challenges the Bardeen-Cooper-Schrieffer theory. To explain
this behaviour, we propose that the superconducting order parameters associated
with the two bands have opposite signs with respect to each other.
