@all,
I recall that some years ago somebody described the implementation of a dual circuit brake system in a 544. My plan is to replace the MC with a new one with two circuits. Does anybody know which MC might be suitable mounting it in the original place? It would be great if I could switch to the 2x3 diagonal circuit, front/rear wolud be suitable as well.
thanks Michael

The early dual circuit split front back and IMHO were no better than the original as long as the original had a functioning hand brake. If you follow the development of dual circuits you will see all types of splits, some of which provide better braking in the event of pressure loss in one circuit. For me, barring two lines to each front caliper most dual line system provided uneven braking when one line was down. A single circuit properly maintained with good hard and flex lines, properly adjusted handbrake, MC with good seals and a warning light for low fluid is all I need. That being said all of the above applies to dual circuit brake systems as well.
In my case I have added a low fluid float scavenged of a scrap vehicle with a cabin warning light in cockpit, if other people drove my car I would add a buzzer.