Why wouldn't you consider the [H+] produce from the weaker acid in a mixture of two acid?
You don't have to consider the system's sub-components separately, but it makes the algebra much easier. 

If pKa of $\ce{HF}$ is 3.14 and pKa of $\ce{PhOH}$ is 9.95 and the problem gives you concentrations to ONE SIGNIFICANT figure, it isn't clear to me why you think the difference between .00072 and .00072000011 is worth worrying about...It can be ignored, clearly. It's not a question of the weaker acid, its a question of whether the values are close enough so that the smaller is significant with regard to the larger. In this case, clearly not. $\ce{PhOH -&gt; PhO- + H+}$ is the chemical equation for the 2nd part, so

$$
\ce{\frac{[PhO][H]}{[PhOH]}} = K_a
$$

is the equation you derive from it (ignoring difference between concentration and activity, and ignoring water). 

The fact that $K_a$ for this is $10^{-10}$ and that $\ce{[H+]}$  is on the order of 0.0007 means you get $\frac{X(0.0007)}{(2-X)}=10^{-10}$ which if you have any done a minimal amount of algebra practice/drill immediately suggests that X is going to be on the order of 3E-7 and again is there really a difference between 2 and 2-0.0000003? You'll note I'm not asking the slightly different question about whether the difference between 2.0000000 and 2.0000000-0.0000003 = 1.9999997; 2 is not the same as 2.0000000 when significant figures are in play.