Work function of aluminium is 4.2 eV. If two photons, each of energy 2.5 eV, are incident on its surface, will the emission of electrons take place?

No, there won't be any photon ejection; no photoelectric effect will take place because the energy of an individual photon is less than the work function (i.e., 2.5 < 4.2). Although the combined energy is greater, only one photon gives energy to an electron.