Has anyone tried using a double UPS for clean power?

If you mean daisy chain UPSs the answer is NO.

I use a UPS on my router to stay connected during power blips. 

If you mean daisy chain UPSs the answer is NO.

I was confused on that point as well - it's NOT daisy chaining two UPS units, which would not be beneficial. It's a single UPS unit designed to internally convert the wall AC to DC, and then from DC back to AC (hence the "double conversion"), with a parallel battery supply in the middle to provide seamless continuity in case of a power outage. Sounds much like a power regenerator, plus the battery backup.

You can say it's not a good idea to run a powerful amp off it, which is true, but many audiophiles advise the same even with powerful regenerators (like the kind PS Audio sells). Plug your amps right into the wall, IMO. 

In the past 40 years I've never had a single device - even the cheapest disposable crap - damaged by a wall surge. I've also had an AQ Niagara 7000 and decided last year I could live without it (I can).

Video is strange- asking a manufacturer for free advice to cheaper alternatives.