i always thought each basher was individual....
so if you had 3 bashers, it would be .85 to the 3rd power which is .61 which means there is a 39% chance to bash every turn. if you also had a 25% chance to bash from a skill, you could add that in by multiplying .61 by .75 to make .45 which gives you a 55% chance of bashing every turn. i've always done my dota items by math and it usually adds up correctly. for the troll warlord, when he is at level 3 rampage, he usually attacks roughly 3-4 times a second and his bash move is 10%. if he had 2 bashers on him, he would have a 35% chance of bashing each turn and after 4 hits he would have a massive 82% chance to bash you every second so i think that the diminishing returns theory is correct because having 6 bashers would give you 90% chance every turn which is pretty much every turn but with DR you only have 56% every turn with 6 bashers.