Allocating tasks among two people equitably
Suppose that the time for completing task $i$ is $a_i$ for $A$ and $b_i$ for $B$. Let $S$ be the set of tasks performed by $B$. The total time required is
$$
\sum_i a_i + \sum_{i \in S} (b_i-a_i).
$$
Your goal is then to find a subset $S \subseteq [2n]$ of size $n$ which maximizes $\sum_{i \in S} (b_i - a_i)$. You take it from here.