How do I identify how many reactions are occurring when precipitating a mixture? An example of a question is: 


  After dissolving $x$ grams of a mixture of $\ce{Na2SO4}$ and $\ce{NaCl}$ in water, you add an excess of $\ce{BaCl2}$ solution. You filter off the precipitate which forms, dry it, and find that it weighs $y$ grams. What is the percent by mass of $\ce{Na2SO4}$ in the original mixture?"


If I am correct, the reaction for this would be:
$$\ce{BaCl2 + Na2SO4 + NaCl -&gt; BaSO4 + 3NaCl}$$

However, there are some questions which are of the same format but have two separate reactions:


  A $x$ gram mixture of $\ce{CaCl2}$ and $\ce{RbCl}$ is dissolved in water. Addition of $\ce{AgNO3}$ causes the chloride ion to precipitate as $\ce{AgCl}$. The mass of $\ce{AgCl}$ collected is $y$. What is the percent by mass of $\ce{RbCl}$ in the original mixture?


For this one, would the reaction be two equations?

\begin{align}
\ce{CaCl2 + AgNO3 &amp;-&gt; Ca(NO3)2 + 2AgCl}\\
\ce{RbCl + AgNO3 &amp;-&gt; RbNO3 + AgCl}
\end{align}

I know how to do these types of questions past the point of deciding whether or not there are two equations. I just don't understand why I couldn't write that last one as a single equation, in the same way as I did the first example. Could anybody help me? Thanks in advance.
You asked two very different questions.


  (1) How do I identify how many reactions are occurring when precipitating a mixture?


There is no information in way the reactants are shown in a chemical equation as to what the products will be. That is to say that you have to know the products of the reaction. 


  (2) For this one, would the reaction be two equations?


Chemists use shorthand all the time. Reactions are typically netted out to the pertinent aspect under consideration. However if I were writing the reactions for the second problem I'd do it much differently that you have presented. I think I'd write:

\begin{align}
x\ce{CaCl2} + y\ce{RbCl &amp;-&gt;} x\ce{Ca^2+} + y\ce{Rb+} + (x+y)\ce{Cl-}\\
x\ce{Ca^2+} + y\ce{Rb+} + (x+y)\ce{Cl-} &amp;\ce{-&gt;[excess AgNO3]} x\ce{Ca^2+} + y\ce{Rb+} + (x+y)\ce{AgCl v}\\
\end{align}

From an analytical chemistry perspective there are two steps. First the chloride salts are dissolved and then the silver nitrate is added. You wouldn't want to add the salts to a silver nitrate solution since the silver chloride could coat the Ca and Rb salts preventing a complete reaction. 

I even might write the second reaction as:

$$ (x+y)\ce{Cl-} \ce{-&gt;[excess AgNO3]} (x+y)\ce{AgCl v}$$

I'm not a fan of one line jobs like:

$$x\ce{CaCl2} + y\ce{RbCl -&gt;} x\ce{Ca^2+} + y\ce{Rb+} + (x+y)\ce{Cl-} \ce{-&gt;[excess AgNO3]} (x+y)\ce{AgCl v}$$