A heater boils a certain quantity of water in time $t_1.$ Another heater boils the same quantity of water in time $t_2.$ If the two heaters are connected in parallel across the same source, the combination will boil the same quantity of water in time:

Let $P_1$ and $P_2$ be the output powers of the two heaters. If $Q$ is the energy required to boil the given quantity of water, then $P_1{t}_1=P_2{t}_2=Q.$ When connected in parallel, the equivalent power output is $P=P_1+P_2.$ Hence the required time t is given by 

$\frac{Q}{t}=\frac{Q}{t_1}+\frac{Q}{t_2}\Rightarrow t=\frac{t_1{t}_2}{t_1+t_2}$