I did split Frontline and do it now with Comfortis too but you can't split Heartguard chewable cubes b/c the medicine is somewhere in that cube and if you break it you never know who gets some protection. You can start with any heartworm prevention before they are 6 months old b/c the life circle of the heart worm isn't completed by then. After that you need to have your baby heartworm tested before you give it to him b/c the medicine could cause him to die if there are heartworms already present. You can use frontline on puppies as young as 12 weeks.
BTW... where in Germany do you live? I'm from Munich...