That's a good point you make actually, every time I've played, the Revive, Speed and Who's Who cola have always been in the Starting Tower; and the Mule Kick, Jug, Double tap and PaP in the Power Tower.
It probably is easier to do it Co-op, but I made these achievement guides to show that they can easily be done Solo. The only reason I go for these on Solo is because you can rape train the zombies without having others possible ruin the route (assuming you all stick together).
I say this, but if the other players are people you know then its all good lol. :P