We report the detection of weak mean longitudinal magnetic fields in the Herbig Ae double-lined spectroscopic binary AK Sco and in the presumed spectroscopic Herbig Ae binary HD 95881 using observations with the High Accuracy Radial velocity Planet Searcher polarimeter (HARPSpol) attached to the European Southern Observatorys (ESO's) 3.6 m telescope. Employing a multi-line singular value decomposition method, we detect a mean longitudinal magnetic field {B_z} = -83 ± 31G in the secondary component of AK Sco on one occasion. For HD 95881, we measure {B_z} = -93 ± 25 G and {B_z} = 105 ± 29 G at two different observing epochs. For all the detections the false alarm probability is smaller than 10-5. For AK Sco system, we discover that accretion diagnostic Na I doublet lines and photospheric lines show intensity variations over the observing nights. The double-lined spectral appearance of HD 95881 is presented here for the first time.