The conductance of a single GaAs quantum point contact (QPC) with in-plane side gates separated from a channel by lithographic trenches is experimentally studied. It is shown that, in such a trench-type QPC, the conducting channel splits into two channels, each demonstrating a conductance quantization. A phase diagram of the 1D subband population in separate channels as a function of the voltages at side gates is experimentally obtained. The obtained data show that the electron transport regime undergoes the transition from the case of two independent parallel 1D conductors to the case of strongly correlated double-row transport. The transition occurs with the increase in the number of populated 1D subbands in the channels and manifests itself in the phase diagram by the change of behavior of the lines separating the regions corresponding to different numbers. The crossings of these lines at small numbers turn into anticrossings at large numbers. The anticrossings are explained by the splitting of degenerate levels in a double-well confinement potential formed inside the QPC constriction.