This concept is rather simple, and I'm sure it's been said before.
The Arsonist should have a similar ability to the Plaguebearer.
E.g.
Arsonist douses Samuel Parris N1.
On N2, Blinky visits Samuel Parris and is also doused. This ensures that more targets are doused quicker.
However, if 2 people visited Samuel Parris, only 1 person would be doused, otherwise, it would be too overpowered.
I hope you take my idea into consideration,
-Myamdane