You absolutely can stack more than 2 - I know of people who have stacked 4-5 at MK for the afternoon/evening.
You could theoretically stack at 7, 11, 1, 3, and 5 and have five evening rides at MK.
When I went a couple of weeks ago I sort of tried this but to book my next G+ I had to have scanned in on my first G+. to book another one.
I think it was Hollywood studios..where I knew I wouldn't be getting into the park till late morning so I grabbed a G+ for Slinky at 4pm....I then booked one once the park opened for Millenium Falcon at 1pm....I tried to book another but it said I had to use my 1pm G+ before I could book another.