Agreed, it can be done, I've done it twice. I suggest you do NO MORE than 2 shows a night. Here are some tips you should follow in order to view 2 shows. Please note, you will need to use Disney transportation in order to use this plan.
VERY IMPORTANT: Don't take ANY bags or purses with you for viewing nighttime shows. It really slows you down because you'll have to go through a security checkpoint before you enter the park which could easily take 10 minutes or more.
The following depend on showtimes
To view IllumiNations then Wishes:
View Illuminations where others have suggested, from the entrance to World Showcase which is pretty much dead center. You'll get a great view and be able to sprint towards the Monorail IMMEDIATELY after the show. (Show is about 15 minute long.) Take the mono over to TTC and then take mono over to MK. Plan to view Wishes from the Main Street train station or the very beginning of Main Street.
To view Fantasmic then IllumiNations:
Sit towards the rear of the theatre to view Fantasmic. (Show is about 22 minutes long.) Exit ASAP and sprint or run towards the park exit. There is a pathway that connects MGM and Epcot. Use that pathway to walk to Epcot. Walk should be about 30 minutes or so, depending on pace. This will take you to the rear entrance of Epcot, the International Gateway. From here, you can find a few good spots to view the show, even minutes before showtime. I suggest the bridge between the UK and France or the lower terrace in the French Island.
To view IllumiNations then Fantasmic:
View IllumiNations from somewhere in the UK or France and immediately after the show exit through the International Gateway and walk at a quick pace towards MGM using the pathway that connects the two. PLEASE NOTE: This only works when Fantasmic is being shown at 10:30 PM since IllumiNations is almost always at 9:00 PM. For the 10:30 showing of Fantasmic, there is always enough seating to view the show, even if you get there right as the show starts.
To view Fantasmic then Wishes:
View Fantasmic from the rear of the theatre and immediately after the show, sprint towards the exit. From there, take a bus from MGM to MK. Plan to watch Wishes from the Main Street train station or from the beginning of Main Street.
Hope that helps, good luck! :wave: