Ask Emtee, his primary broke up, couldn't exit via the secondary's as it blocked it, got sucked back into the engine as a sand, mixed with the oil, game over, scrap engine.
This was an issue with the Euro spec 4200 and early GS's with the two cats in one pipe. Maserati remidied it by using header cats then secondary's further down the system which remedied the cat break up choking and ingestion.
Arnies GS has the benefit of free flow secondary's which will further allow any primary break up to exit out the back, but for me, one pair of cats is enough, secondary's are just not needed and are only there at the request of the engine manufacturer to stiffer the engine output...the same as the squish pipes do.