You can highlight both stereo objects, then right click on one of them one click "group objects" (it might be "link objects", I'm not in front of Samplitude right now. But I think it's "group object".) Anyway, once the two objects are grouped together (or linked together), if you grab one of the handle grips to slide the beginning/end position of the object, you'll see that the beginning/end position of both objects move simultaneously (because they're grouped together). I think this will allow you do to what you want, but if not, please clarify exactly what you are trying to accomplish.
- Jason