You'd have one 340 square inch cage with a 340 square inch modular addition,not a 680 square inch cage.Because of it being a modular addition it cannot be considered in the actual floor space of the single cage,which needs to be kept in mind.
Do keep in mind that modular cages and modular additions are not hamster-appropriate,and because of the nature of pipes and how terrible the ventilation is it can cause serious health problems.Pipes,even just one or two,can be terrible for breathing if nested in and then urinated in as well.Even large pipes can be filled and urinated in,and no matter how large the pipe is it will still have inappropriate ventilation
