You would appear to be in the middle of the L/ and the XL/ sizing spec. with a slight bias towards XL. However, my "guess" without seeing the set-up on your personal bike (reach/handlebar position relative to seat & seat height/set back) is that you could make due with this size tandem as a first tandem. You would most likely need to get a longer stem with the appropriate rise to correct the handlebar position relative to your seat height and you may need a longer than stock seat post to make up for the 3" difference in seat tube length.
Again, this is just chin scratching based on the numbers. You'd really need to check the net seat height between your current rig and the dimensions on the C'dale to assess how much seat post mast will be exposed as well as the reach from your saddle nose to the handlebars to assess the probable stem length. If you find that you'd have an inordinate amount of seat post mast exposed or that you needed an exceptionally long stem to correct your reach then you'd probably need the next size up.
No guarantee or warranty of this information is implied, but I did sleep at a Holiday Inn Express last night....