Not really.

You want to calculate the probability to find all the molecules in the left half of the volume.

Since the probability density is 

$$\rho(\{q,p\}) = c_N \frac{e^{-\beta H(\{q,p\})}}{\Omega(U,V,N)}\ ,$$

where, to ensure the normalization, $$\Omega(U,V,N ) = c_N\int e^{-\beta H(\{q,p\})} dq^{3N} dp^{3N}$$

what you need to consider is

$$\text{Pr} =\int_{\{q,p\}_{V/2}}  \rho (\{q,p\}) dq^{3N} dp^{3N} = c_N \frac{ \int_{\{q,p\}_{V/2}}  e^{-\beta H(\{q,p\})} dq^{3N} dp^{3N}}{\Omega(U,V,N)}$$

where $\{q,p\}_{V/2}$ is the set of coordinates such that all the molecules are in the left half of the volume.

You can immediately see that 

$$c_N  \int_{\{q,p\}_{V/2}}  e^{-\beta H(\{q,p\})} dq^{3N} dp^{3N} = \Omega(N,V/2,U)$$

So that you finally obtain the desired result.