Probability that all bins contain strictly more than one ball? Here's the problem I'm working on:
Given that I'm distributing $N$ balls into $K$ bins, what is the probability that all bins contain at least two (strictly more than 1) balls? This seems like a very similar question to asking what the probability that all bins contain strictly more than zero (e.g., all are occupied), but for whatever reason it's a lot harder!
Although the problem is similar to this related question addressed by Henry, Joe, David Mitra, and MJD regarding the expected number of bins containing >1 balls, it doesn't seem like I can apply the same method since different occupancy sets occur with different probabilities.
For example, distributing $N=4$ balls into $K=2$ bins, there are $K^N=16$ total distributions, six of which have two balls in each bin, giving me a probability of $6/16$.
Is there a general solution?
 A: The probability is 
$$\frac{K!}{K^N} \sum_i (-1)^i \binom{N}{N-i} \left\{ N-i \atop K - i \right\},$$
where $\left\{ n \atop k \right\}$ is a Stirling number of the second kind.
The $S_2(N,K)$ I have below satisfy $S_2(N,K) = K! \, T(N,K)$, where $T(n,k)$ is a 2-associated Stirling number of the second kind.  (See also their OEIS entry.)  The $r$-associated Stirling numbers of the second kind are the number of ways to partition a set of $n$ objects into $k$ subsets so that each subset contains at least $r$ objects.  The subsets can be considered as indistinguishable urns, so to distinguish them we multiply by the number of ways to order them (i.e., $K!$) to get $S_2(N,K)$.
There is a known formula for the 2-associated Stirling numbers of the second kind.  It's $$T(n,k) = \sum_i (-1)^i \binom{n}{n-i} \left\{ k-i \atop k - i \right\}.$$  See, for example, Fekete, "Two Notes on Notation, American Mathematical Monthly 101(8): 1994, pp. 771-778.  (My apologies for the JSTOR link.)
Since the probability is $$\frac{S_2(N,K)}{K^N},$$ we get the result.  So my comments below were overly pessimistic.

Original answer:
Charalambides's Enumerative Combinatorics, Exercise 9.23, says, "Let $S_r(n,k)$ be the number of distributions of $n$ distinguishable balls into $k$ distinguishable urns so that each urn contains at least $r$ balls."  The OP is asking for $$\frac{S_2(N,K)}{K^N}.$$
For the $r=2$ case, the exercise asks to prove the generating function $$S_{k,2}(t) = \sum_{n=2k}^{\infty} S_2 (n,k) \frac{t^n}{n!} = \left(e^t-1-t\right)^k.$$
The exercise also gives the recurrence relation 
$$\begin{align}
S_2(n+1,k) &= k \bigg( S_2(n,k) + n S_2 (n-1, k-1) \bigg), \:\: n \geq 2k, \\
S_2(n,k) &= 0, \:\: n < 2k, \\
S_2(2k,k) &= \frac{(2k)!}{2^k}.
\end{align}
$$
The fact that Charalambides does not include an explicit expression is not a good sign.  The generating function and the recurrence relation may be the best we can hope for (especially since either differentiating the generating function $n$ times or unrolling the recurrence looks to be difficult).
Added: The $S_2(N,K)$ numbers are sequence A200091 in the OEIS.  There's nothing there beyond what's in Exercise 9.23 in Charalambides's book. 
A: I would think the following approach should work. Let $A$ be the event "There is at least one ball in each bin" and $B$ the event "There are at least two balls in each bin". Then $P(B) = P(A)\cdot P(B|A)$, since $B$ cannot occur without $A$.
You seem to have already calculated $P(A)$, and $P(B|A)$ is calculated exactly the same way as $P(A)$, only with $N-K$ balls, since there is already one ball in each bin.
A: I believe you can simply start off by taking away the $2K$ balls that are evenly distributed among the $K$ bins and then you are left with just counting the number of ways to assign $N-2K$ balls to $K$ bins. Divide this by the number of total ways to assign $N$ balls to $K$ bins and you have your answer, namely: $$ \frac{K^{N-2K}}{K^N}$$
A: This is equivalent to  solve
\begin{equation}
x_1 + x_2 + \cdots +x_k = N
\end{equation}
with $ x_i \ge 2, i=1, \cdots k $ is integer nonnegative. Also this is equivalent to  solve
\begin{equation}
y_1 + y_2 + \cdots +y_k = N
\end{equation}
where $ y_i = x_i + 1.$ Then this is equivalent to  solve
\begin{equation}
x_1 + x_2 + \cdots +x_k = N - k
\end{equation}
where $ x_i \ge 1 $. Clearly we can assume that $ N-k \ge k $, otherwise the probability is zero. Thus the anwer is 
$ \binom{N-k-1}{k-1} $ because each solution can be represented by put $ k-1 $ traces beteween $ N-k-1 $ places to separate the balls.
The probability is 
\begin{equation}
\dfrac{\binom{N-k-1}{k-1}}{\binom{N+k}{N}}
\end{equation}
because the number of ways to put $N$ balls in $k$ bins are $\binom{N+k}{N}$ (the number of ways of put $N$ balls and $k$ traces).
