Content is only installed once and is shared between all Steinberg plugins that can use it. If, for example, you install an add-on that can be used in HALion Sonic SE, such as Olympus Choir Micro (chargeable) or Alto Glockenspiel (free - you just have to request a licence code), you install the content once and it appears in HALion, HALion Sonic and HALion Sonic SE if you have all three installed. Similarly, Groove Agent add-ons appear in both Groove Agent and Groove Agent SE if you have both installed.
You can use Steinberg Library Manager to manage this shared content. Steinberg have finally upgraded the latest versions of all their plugins to use this system - Padshop 2 and the latest builds of The Grand 3 and Retrologue 2 add Library Manager support.
HALion Sonic is installed alongside HALion to allow HALion users to open projects that use HALion Sonic. Sometimes the simpler HALion Sonic interface is all that is needed (e.g. live performance scenarios).