We report the magnetic properties of the double perovskites SrLaCuSbO6 (SLCSO) and SrLaCuNbO6 (SLCNO). The temperature dependence of the magnetic susceptibilities of both compounds shows a broad maximum characteristic of an S=1/2 square lattice Heisenberg antiferromagnet. Magnetic ordering occurs at TN=13.6 and 15.7 K for SLCSO and SLCNO, respectively. Neutron powder diffraction measurements reveal contrasting spin structures in both compounds. The spin structures of SLCSO and SLCNO below TN are Néel antiferromagnetic and columnar antiferromagnetic, respectively. This result demonstrates that the nearest-neighbor interaction is dominant in SLCSO, whereas the next-nearest-neighbor interaction is dominant in SLCNO. The magnitude of the ordered moment was evaluated at 3.5 K to be m=0.39(3)μB for SLCSO and 0.37(1)μB for SLCNO, which are significantly smaller than those calculated using linear spin wave theory. We infer that the small ordered moment is caused by the effect of bond randomness arising from the site disorder of Sr and La ions.