The reason you have spent a lot of time looking w/no success is b/c you are looking for something that really doesn't exist here. Did you search using "venere.com"

Italian hotels charge by the person, not by the room. Before you start thinking, we'll book and double and simply put a 3rd person in the room, you must submit passports to check in (its the law) and the clerks catch on when you are trying to get around it.

You might try looking for two rooms, one single and one double but I do not believe you will be able to find accommodations in Venice proper that will accommodate all of your requirements AND remain in your price range (at least not at the current exchange rate, if the $$ continues to strengthen, your options will increase somewhat - but just like Manhattan, London, etc.. hotel rooms are expensive).