This study introduces a side-by-side phoswich PET detector consisting of currently widely used LYSO:Ce and recently newly developed GAGG:Ce for providing high spatial resolution of <0.4 mm. A Monte Carlo simulation was conducted to investigate the light collection efficiency (LCE) and the imaging performance of the proposed design. Crystal block consists of crystal assembly of 17x17 array arranged with a 0.9 mm pitch, and has a 15.5x15.8x10 mm3 volume. Each crystal assembly is consisted of a phoswich pair assembled with LYSO (0.4x0.8x10 mm3) and GAGG (0.4x0.8x10 mm3). They were optically coupled along one long side and covered with reflective film to other side surfaces and top surfaces. The X-Y position of each crystal assembly can be distinguished by a high reflectivity material and the specific interaction position of different crystals within phoswich pair was distinguished by pulse shape discrimination method. The 8x8 GAPD array is comprised of 1 x 1 mm2 active area pixels arranged with a 2.0 mm pitch. The monolithic light guide was inserted with the optical grease between the crystal block and GAPD array, to distribute the scintillation light on the GAPD sensitive area. LCE value in the proposed crystal assembly was similar for LYSO and was improved from 26.7% to 34.6% for GAGG, compared to conventional high resolution PET detector designs with relatively narrow pixelated crystal. It should be noted that LCE value of 34.6% in the proposed crystal assembly with 0.4 mm pixel width was quite better than 29.6% for the relatively wide pixelated GAGG of 0.8 mm pixel width. Also, the flood histogram without the overlapping of each crystal position can be generated for side-by-side phoswich PET detectors using light guides with a thickness of 1-2 mm. This study demonstrated the proposed design will provide opportunities to develop PET detector with high spatial resolution of <0.4 mm.