Catalyzes the transfer of the mannosyl group from GDP-mannose to di-myo-inositol-1,3'-phosphate (DIP), producing mannosyl-di-myo-inositol phosphate (MDIP) (PubMed:19648237). Can also use MDIP as an acceptor of a second mannose residue, yielding di-mannosyl-di-myo-inositol phosphate (MMDIP) (By similarity). bis(myo-inositol) 1,3'-phosphate + GDP-alpha-D-mannose = 2-O-(beta-D-mannosyl)-bis(myo-inositol) 1,3'-phosphate + GDP + H(+) 2-O-(beta-D-mannosyl)-bis(myo-inositol) 1,3'-phosphate + GDP-alpha-D-mannose = 2-O-(beta-D-mannosyl-(1->2)-beta-D-mannosyl)-bis(myo-inositol) 1,3'-phosphate + GDP + H(+) bis(myo-inositol) 1,3'-phosphate + 2 GDP-alpha-D-mannose = 2-O-(beta-D-mannosyl-(1->2)-beta-D-mannosyl)-bis(myo-inositol) 1,3'-phosphate + 2 GDP + 2 H(+) Belongs to the MDIP synthase family.