I suppose it depends upon what all you want to see & do.
We like to plan on 2 days there, and maybe even another evening.
What has worked for us in the past is to spend the mornings in FW doing rides & Innoventions, then head over to WS for the afternoon, for lunch or dinner, or both. We enjoy eating around the world and seeing the shows & musicians, etc. On longer trips, if we have a DTD or a pool day, we'll sometimes go into WS for dinner and just to stroll around for the evening.
Or another way we've enjoyed doing it is to spend a day in FW (since it seems lots of folks use the above strategy, so some of the FW rides get shorter lines in the afternoon), then sleep in for a 2nd Epcot day in WS (since it opens later).
Either way can be enjoyable, depending upon your preference. WS is one of our favorite places in the whole world!