Dear Paolo, the main issue is: The whole concept of MIR relies on Ambisonics. It gives us the possibility to define many acoustic parameters after the actual recording took place, and - equally important! - we can interpolate seamlessly between discrete source positions.
An Ambisonics recording is typically based on a so-called "coincident" multi-microphone array. We used indeed up to four of these arrays to capture the Venues represented by MIR - which is why we can add the Secondary Microphone, but that's for the wet signal component only: The goal is to increase the perceived spatial enveloping without sacrificing any of the positioning cues of the Main Mic. And yes, it's a great source for decorrelated rear mics in a surround mix, too!
The typical Synchron recordings use a completely different approach, based on wide-spaced single microphone capsules. They are referred to as "Decca tree", supported by Omni-outriggers and AURO top-layer mics. In short: A very different approach than a coincident recording. ... there have been lengthy discussions amongst Tonmeisters during the last 70 years (or so) which approach is "better", so I won't go into the details here. ;-)
Although I went to any length in my quest to match the sound of the Synchron Instruments' setup for VSL's "Synchronized" Vienna Instruments and now for MIR/MIRx, it has to be said quite clearly that there won't (and can't) be a MIR engine as we know it that will be able to fully recreate a spaced microphone setup as described above.
/Dietz - Vienna Symphonic Library