dude, the Periscope Large is also a 22.5 front toptube. Way too small. Toptube measure is teh way you determine fit, not the height of the saddle. Otherwise, the geometry between you and the drivetrain won't be right. Seat height is adjustable.
You gotta start with a good fit. As an example, the Cannondale XL/S tandem has a 24cm front toptube. That's alot closer to what you'd need. I don't want to sound like a Cannondale shill, but for $2000 they're a good place to start tandeming. And you'd be in the right size.