Backwater Vavle: 2 layers of protection desired

Hello -

I'm looking for some advice/second-third opinions and appreciate any!

Last year our neighborhood experienced two sewer backups due to some (putting it nicely) unknowledgeable neighbors flushing wipes....We live in the house with the lowest drain on the street...so you know where this is leading.

The City installed one of the basic cast iron check-valves and paid for restoration, and when those neighbors continued flushing wipes, another event occurred, and that check-valve failed. Yeah... Pissed, upset, angry, you name it, emotions ran rampant.

So...we undertook a campaign to educate everyone about what not to flush, included the City to make it official, etc., and we had a Mainline ML-FR4 backwater valve http://backwatervalve.com/index.html installed (we have limited slope on our sewer line) and while we understand this is a far superior valve, we cannot get past wanting to have more then one layer of protection, over-engineer the solution, ie, not have a single point of failure should something happen again.

The ML-FR4 is installed near the house and is 60 feet from the street.
The lowest drain is below the washout at the ML-FR4.

So, we're considering adding another valve somewhere to hopefully foolproof it, but haven't yet figured out exactly where and what type of valve would do the trick. Thoughts are something like the Mainline Straight-Fit.

Appreciate any guidance on this!
RNB