I'd assume if they're marketing both at the same time, the dedicated broadhead sharpener is likely their only solution for them. Having said that, I'd think some improvisation with the knife sharpener might be possible to at least put a useable edge on broadheads, if not an excellent one. If the dedicated knife sharpener's clamp is capable of holding smaller and narrower knife blades (still able to access a blade edge close to the clamp's front edge, in other words), I'd think it should be able to clamp and hold a broadhead as well.
If it were me, I'd just call them and ask if the knife system can do it by itself, and/or what limitations it may have in doing so. They might've engineered the dedicated broadhead sharpener to cover an option or functionality that the knife sharpener can't reliably handle by itself. Only way to know, is to ask them.
David