No time to go and check the books, but I think that if you had one engine out, you would not want to use hot air from the other engine(s) to anti-ice that dead engine*. That would be a waste of power, which would not be desirable with one engine out.
But obviously you would never want to anti-ice only one wing !
*do engines only anti-ice themselves or does each engine contribute to anti-icing all engines ? Can't remember.