You could do this spilloveralgorithm, just like Strymon does. Sorry to mention them all of the time, but they have a 5 second rule between switching patches. They sample (i e like recording into a looper, but… not actually looping it) the current patch trails of reverbs or delays, so it actually trails, and sounds, even if you switch to a completely different patch. So whenever you switch a patch, the spillover effect is actually a recording, or “copy print” or “screen print” of the latest notes and effects, that got into the signal chain processed, of the LAST 5 SECONDS. Now, whenever you start to play with the new patch, the old will still sound one cycle around (5 second) in the background, at a lower level. The rule you must keep in mind is that you can’t switch presets in two successions in a row (well you can…but*), because then the 5 second “recording” of the old one will not have had time to fulfill its complete cycle. I e not change more than ONE preset at a time WITHIN 5 seconds. So as long as your delay trails are of max 3 seconds, and the looper is, like, 30 seconds, what’s stopping you?
*Well, you CAN switch presets to how many as you can within 5 seconds, but don’t expect spillover then.