Peptide bonds are covalent bonds. Disulfide bonds are also covalent bonds. Hydrogen bonds are not covalent - they are inter or intra molecular forces. The reason why disulfide bonds are stronger has nothing to do with whether they are between two different proteins (you might consider this intermolecular - I think would just make them one big molecule, but it really depends on your perspective) or are between two amino acid residues of the same protein (intramolecular, but again I personally don't see it that way). Covalent bonds are ALWAYS stronger than hydrogen bonds.