How to find the Utility Possibility Frontier when there are Perfect Substitutes?
In addition to the solution worked out in the comments, there's also a faster way to find the welfare maximizing distribution:

Note that social welfare can also be written in terms of $x_{1A}$ and $x_{2A}$ as $W(x_{1A},x_{2A})=15+5x_{1A}+x_{2A}$, which is maximized in the Edgeworth box at $(x_{1A},x_{2A})=(5,5)$. As long as utilities are increasing in the quantities of both goods, this point is always on the contract curve and is therefore the maximizer of social welfare independently of the specific forms of the individuals' preferences.