I think you'll find that the class 5 is also a siphon jet design...they can call it whatever they want, but it seems they made up their own classification system...class 5 has no industry standard and is something marketing came up with implying it's wonderful...and maybe 5x better than anything else out there...it isn't!
Generally, while you may be able to cobble up a dual flush system, it may not be the best idea - on a well designed one, the bowl would be different. On a dual flush system, you still want the bowl to be emptied, regardless of the volume of water used. To do that, they tend to have less water in them so you can empty it well on the lower-volume flush otherwise, you'd just be diluting it, not emptying the bowl.
IOW, to get it to work properly, you're much better off buying one designed to work as a dual-flush, not cobble one up with questionable results.
