Black Streaks on Gutters

Has anyone tried using a white scrub pad, or something similar to remove the most stubborn black streaks. Sometimes gutter stains are a bit difficult to remove even when using a strong gutter cleaner and a soft brush.

Two gun System vs. One gun System

I saw an advertisement from Envirospec where they had a special price for a dual system, 30 Hp. Kohler with pump, that produces 5 gal. 3500 psi for two users or 10 gal. for one user. What are the pros and cons for using a duel system vs. using two separate engines....