Heterostructures of transition metal oxides perovskites represent an ideal platform to explore exotic phenomena involving the complex interplay between the spin, charge, orbital and lattice degrees of freedom available in these compounds. At the interface between such materials, this interplay can lead to phenomena that are present in none of the original constituents such as the formation of the interfacial two-dimensional electron system (2DES) discovered at the LaAlO3 /SrTiO 3 (LAO/STO) interface. In samples prepared by growing a LAO layer onto a STO substrate, the 2DES is confined in a band bending potential well, whose width is set by the interface charge density and the STO dielectric properties, and determines the electronic band structure. Growing LAO (2 nm) /STO (x nm)/LAO (2 nm) heterostructures on STO substrates allows us to control the extension of the confining potential of the top 2DES via the thickness of the STO layer. In such samples, we explore the electronic structure trend under an increase of the confining potential with using soft x-ray angle-resolved photoemission spectroscopy combined with ab initio calculations. The results indicate that varying the thickness of the STO film modifies the quantization of the 3d t_2g bands and, interestingly, redistributes the charge between the d_xy and d_xz/d_yz bands.