AudioDirector can be invoked directly from any timeline clip by right-clicking on it and choosing AudioDirector, which is the best tool for this task. It also shows a video preview which may help with syncing the pan to the video content:
You can also open PD's built-in Audio Editor the same way, and while it lets you work on the individual audio channels independently, there isn't an easy way to pan.
If you really want to stay in PD, you could use A E to mute one channel then copy the clip and mute the other so you'd then be able to use volume keyframes as needed to pan across them with them both synced on the timeline.