fn = function(data, arg1 = c("fn1", "fn2"), arg2){
  #does something

  #then calls
  mapply(function(a, b){
    doCall(.fcn = a, data = data, arg = b), arg1, arg2
  })
}

fn1 = function(data, arg){
  #does some long computation which is parallelized
}

fn2 = function(data, arg){
  #does some long computation which is parallelized
}