I have a feeling the noise you heard was the resonance frequency of the fans interacting.If I knew for sure that one fan wouldn't hinder the other enough to make it heat up to the point where it could be dangerous, then I'd do it straight away, but neither of my fans ever made any noise like it did when I tried both. I just thought because it was making noise something was wrong, it was like a whine.
I might fire off an email to a manufacturer to see what they think.
I can understand that it will boost air pressure within the duct which in turn will get more air through. This fan works fine for 3 lights, adding 2 more in a straight run with cool-tubes is adding more friction so yes that could well be the problem, I just need to know for sure that extra fans on the same run isn't gonna be a fire hazard.
I think the poster gave you sound advice.
It absolutely is a good remedy along with gentle bends in the ducting not elbows.
Lots of little improvements give significant results.