Ahhh, now I understand which "dual exhaust" you meant. Sorry, my first thought was you were looking for a manifold. But that's why I asked.
Those quad Anza's (pic below) are getting difficult to find anymore, as you have found. And if you do locate one they aren't as cheap as they used to be. They look great - classic sports car style (Maserati pic below). But as much as I love the appearance of the Anza exhausts, I've found them to be somewhat short lived. They seem to blow out their packing fairly quickly. And they will rust fairly bad as well. Sad because they have a great sound to go with the look (until they blow out). Aside from any performance aspect, I'm not sure they are worth the money for the quality.
At one time there was a very similar exhaust system made for the old air cooled VW bugs. They were easy to get and really cheap. So X1/9 guys were modifying the muffler to fit with a Fiat exhaust. Unfortunately they were extremely restrictive (much worse than any X exhaust) and a really bad idea. But it gave the look with little cost.
As an alternative take a look at what a few members have done with a aftermarket muffler that has one inlet and two outlets. It's laid out with the inlet and one outlet on one end, and another outlet on the other end (pic below). They are designed to mount transverse so the two outlets can lineup to make two exhaust tips out the rear of the X. There are a few mufflers with this design. A common one is made for the older Camaro, but there are aftermarket ones that are more performance oriented.
@mkmini did a nice job with his, as have others. Naturally this will require fabrication of the system to fit it to the manifold and out the tail. See if you can find some of the threads with those aftermarket mufflers on X's to see how they were done.
If you really want the Anza quad tip look there is another option. You can build a twin outlet system like I just described above, then add the Anza style dual tips. They are available as a accessory item made by different exhaust companies (pic below). That way you could get them with a larger ID tube to help with the flow.
Some pics to help with the dreams:
Here is the one I believe you are seeking...
Here is the VW version (which might still be available, but not recommended)...
Here is a aftermarket X1/9 header system with a dual outlet muffler. The muffler on this one is similar to what I described above, with two outlets...
I think this is kinda the look you are after...
Here's an example of the generic aftermarket "Anza style" dual tips that can be purchased in various sizes. A pair of them could be added to a dual outlet muffler (example of the muffler below)...
This is the Camaro muffler. There are others with similar designs...
Regarding any dual outlet exhausts on a X1/9. Someone once mentioned they thought twin exhaust tips were not appropriate for the X because the X doesn't have two exhaust pipes like a V8 or V12 engined vehicle does. That makes absolutely no sense to me. It's like saying you can only have as many tips are there are cylinders, or heads, or engines in the vehicle. One has nothing to do with the other. Purely from a 'style' perspective it is completely subjective and therefore anything goes. So build whatever you like for your car. In my opinion the more unique you make your X the better.