Tires aren't load rated differently for a single/tandem/triaxle application.
The two different ratings are for dual application. Some folks mistakenly call a tandem axle trailer ...a dual axle trailer. My Sailuns on my triaxle GN trailer have a 129/125 single/dual load rating.
Lots of tandem dual axle trailers on the road. I have one sitting in the trailer shed. Don't forget ST tires = special trailer aren't just for a rv trailer.
JMO experience in dealing with tire stores on a corner when I was on the road...wording such as "tire experts" and "tire dealers" shouldn't be used in the same sentence especially where it come to their knowledge of tires for trailers.