[Custom Feature Question] Togglable double fishing battles

im currently trying to set up an "attachment" for fishing that makes the fishing encounters double battles; this just works with toggling a switch no issues there.

however its actually getting the double battles to occure.. since the items themselves cause the encounters i cant do it via field or encountermodifiers (atleast not in my attempts) so kinda stuck now.

i am using 16.2 for reference.