The APU will autostart in a TEFO situation, the RAT or APU are only required in loss of both engines (TEFO) or loss of main electrical system, ALL 4 busbars.
Loss of 2 VFG's the FCOM says 'consider' the APU. You can actually dispatch with 2 GEN's inop (1 per side).
So no need to start the APU at all.