Answer
The threshold frequency would be halved.
Work Step by Step
In the case of an exceedingly rare event where an electron simultaneously absorbs 2 identical photons and escapes the material, each of those photons would only have half the energy of a single photon in the regular photoelectric effect process.
Photon energy is proportional to frequency, so the threshold frequency would be halved.
Equation 38.4 would be modified to read $eV_o=2hf-\phi$.
