I tried these x blades on mine.
first issue is finding a blade bolt that will work. It's a weird bolt, has a 30mm head, but M12-1.25 threads. I tried several different kinds with a standard size head but nothing will really "work" with the way the washers and such are designed, other than a factory-style bolt.
The second issue (and this is also important) is that having twice as many blades means twice as much rotating mass. Now mine is just 27hp gas engine, but makes the same power as the diesels do, just uses more fuel doing it. It struggled to turn all SIX blades when in heavier grass. Turning the blades on, it was immediately apparent that the clutch was not going to last long. In my case, because it's an electric clutch on the crankshaft, it's easily replaced-and somewhat inexpensive (aftermarket), in the case of the ZD's and bigger ZG's, that is not the same truth-those have internal hydraulic clutches which are no fun, nor cheap to have to service.
I used mine two mowings. The cut quality was better. The second mowing I was running along just fine and one of the blade bolts failed to hold the blade in place, so one spun, the bolt broke, and the two blades on that spindle came off, then of course the others crashed into it, beat the daylights out of the mulching baffles, the bottom of the deck, broke a spindle housing, and made a whole lot of racket and vibration. So far that's all I found wrong. OH wait I did find that the engine mount bolts were loosened. 3 of the 4 were missing, one broken completely off and the only one left was loose. Muffler was dangling as well and blew out the gaskets.
So no, based on my experience, those X blades are not worth the money. Maybe you can source some bolts that fit properly, with the correct bolt head size, shoulder length, thread length. Basically, you want a factory bolt that is about 1/4" longer thread and maybe a little more shoulder. I couldn't find any with the correct dimensions. Maybe you can have a machine shop make them for you, shouldn't be too hard. How expensive they'll be, is to be determined. I wouldn't be surprised if they hit you with $400 for 3 blade bolts based on this area's shop costs.