Heavy duty gimmick. Two cans with separate compartments and rotating mouths. It'd be a b***h to construct, but i'm sure it could be done. It'd have one side smaller than the other side, and have the smaller side filled with the original drink from the can. The opposite drink in the bigger side, so if you pour the drink out, it would look like almost like a full can, giving a better illusion than if the cans were half and half. So you'd open both cans, pour a little out, to show it's just the regular drink. Rotate the tops (secretly of course), and empty the cans into a glass.
I'm also thinking of the division in the can's contents being horizontal instead, but this is all just me typing while I think.