Hi gyp is 4 yrs and Kai 3yrs old same parent they have both been neutered the behaviour has been there a couple of years it happens when there off lead . I’m lucky as I have just retired so they have no need for doggy day care . Apart from pulling them apart or calling there names I don’t know what to do .they have no favourites with dogs luckily most of the dog owners are nice but one day they may not be I hope you have enough information for the moment thanks Jackie