A brute force approach shows that at least 4 out the 5 biggest stacks will seat at the same table about ~1.9 times out of 100, if I understood your question correctly.
There are 42 players. For each of them, you select a position between 1 and 42. I assume that positions 1-9 go to table 1, 10-18 to table 2 and the other 3 tables will be 8-handed. Here is the code in R that enumerates any possible position combo for the top 5 and counts the maximum number of top stacks at the same table:
Code:
top5<-combn(42,5)
tables<-matrix(rep(1:5,c(9,9,8,8,8))[top5],nrow=5)
howmany<-apply(tables,2,function(x) max(tabulate(x)))
setNames(tabulate(howmany)/length(howmany),1:5)
1 2 3 4 5
0.0487522747 0.7175161167 0.2150686284 0.0181692505 0.0004937296
