Evolution: How could all useful traits evolve simultaneously?
Has anyone calculated or simulated how the adaptation for many different traits can happen simultaneously?


There are a lots of studies on the subject but I don't fully understand what is your issue. So I'll try to give some words hoping that helps a bit but it is possible that I'll totally miss the point you want to make.

The mutation rate in humans is around $1.25 \cdot 10^{-8}$ per base pair per individual per generation. The human genome contains about 3400 Mpb, meaning that you probably carry $1.25 \cdot 10^{-8} * 3.4*10^9 = 42.5$ mutations that none of your parents had in their genome. Now if you consider the population size, you'll see that a fairly big amount of mutations occurs each generation in the human population. So mutations are not that rare! It might unfair to take human as an example though as human's population size is very big. Anyway, the great majority of mutations are deleterious. In consequence, you are right, probably not many beneficial mutations will occur in a given population at one given generation and especially not in the same individual (except if you think of viruses!).

Now, depending on how beneficial is the mutation you are considering (selection coefficient), depending on its dominance, depending on the (effective) population size, your new mutations might take quite a bit of generations before reaching fixation in the population. In this time other mutations might occur. And then, the question is very interesting. Imagine a population of asexual individuals where two beneficial mutations exist (at two different loci [=position on a chromosome]). Because there is no recombination (by definition of what is an asexual population) never these two mutations will be found in the same individuals and in consequence if one mutation reaches fixation, it necessarily mean that the other mutation would have disappear. In sexual population it is different. Recombination allow that the two mutations, once they reaches a sufficient frequency will probably be found in the same organism and therefore, the two mutations can reach fixation.

Hope that helped a bit