Simplicial approximation for simplicial spaces  Given two simplicial topological spaces $X_{\bullet}$ and $Y_{\bullet}$ (i.e. a simplicial object in Top) and a continuous map between their geometric realizations $f \colon \lvert X_{\bullet} \rvert \to \lvert Y_{\bullet} \rvert$. Is $f$ homotopic to $\lvert \varphi_{\bullet} \rvert$ for a map $\varphi_{\bullet}$ of simplicial spaces?
 A: The answer is no. For an arbitrary simplicial space $X_\bullet$ we can consider $|X_\bullet|$ as a constant simplicial space, lets call this $Y_\bullet$. Then there is clearly the identity $|X_\bullet| \to |Y_\bullet|$, but there is in general no nontrivial map $X_\bullet \to Y_\bullet$ (take e.g. BG for a top. group $G$).
A: I'm just reformulating your question in simplicial case. 
If you consider the category of simplicial sets $\mathbf{sSet}$ you can formulate your question as follows: 
Is the diagonal functor $diag: [\mathbf{\Delta}^{op},\mathbf{sSet}]\rightarrow \mathbf{sSet}$ from bisimplicial sets to simplicial sets homotopicaly full ? 
The diagonal functor is actually the realization functor. If we put the diagonal model structure on $ [\mathbf{\Delta}^{op},\mathbf{sSet}]$ then $diag$ is a Quillen equivalence.
As a consequence, if $f: diag(X_{\bullet, \bullet})\rightarrow diag(Y_{\bullet, \bullet})$ 
 is a bisimplicial map and $X_{\bullet, \bullet}$, $Y_{\bullet, \bullet}$ are fibrant-cofibrant bisimplicial sets (in the diagonal model structure), then there exists $g: X_{\bullet, \bullet}\rightarrow Y_{\bullet, \bullet}$ such that $diag(g)$ is homotopic to $f$. 
