Purely hypothetical.
That's an interesting synopsis from 'skiesfull'.
But with respect to the original question: a sudden loss of two engines (on the same side?) I think the concern would be more towards the loss of directional control as the other two engines are developing near max thrust. In fact a thrust reduction *might* be in order.
In the case of two symmetrical engine failures, well, in that case you're not concerned with directional control. I submit that firewall thrust would be acceptable while realizing that any engine will exceed its limitation for a certain length of time. Those engines may never get used again, but at least maybe you can return in one piece.