You’re going to have a hard time then. Your two instances could be running in different sandboxes, or even if they’re in the same process, there’s no guarantee that during each process block their callback methods will happen in the same order.
e.g. in many hosts (certainly in Tracktion this is true) there’ll be a thread pool / work queue where parallel CPU cores pull the next item to process. So the order in which two plugins get their process callbacks executed is definitely not something you can rely on to stay constant. So it’s not possible to just pass a block from one instance to the next each time, you’d need to build in fifos and other nastiness to stream the audio, adding latency.