Two quasilinear utility functions Suppose there are two agents $\{A,B\}$ with the following utility functions: $U_A(x_A,y_A)=x_A+f(y_A)$ and $U_B(x_B,y_B)=x_B+g(y_B)$

$f$ and $g$ are strictly increasing and strictly concave. How do I prove that in the set of pareto efficient allocations $y_A=y_B$ ?
The statement is not true. 
Let $x_A + x_B = 1$, $y_A + y_B = 1$. 
Let $U_A(x_A,y_A) = x_A + \ln(y_A)$, $U_B(x_B,y_B) = x_B + \ln(y_B + 1)$. 
$f \neq g$ are both strictly increasing &amp; concave. For all $z \in [0,1]$ the distributions 
$$
(x_A,y_A) = (z,1), \ (x_B,y_B) = (1-z,0).
$$
are Pareto-efficient. This follows from $MRS_A(x_A,y_A) \geq MRS_B(x_B,y_B)$. 
 In these cases $y_A \neq y_B$.