Congrats on wanting to get a tandem! Buying a used tandem is a great way to get started.
Since bike geometry can be so different, it is really hard to tell what you are in for. 56 cm and 23 in really doesn't tell you a whole lot. For the captain, what you really want to know for both bikes is stack and reach.
There is a good explanation of why this is important here
. It is pretty easy to get these measurements with just a tape measure, a plumb bob (or any piece of string with a heavy object on it), and a little basic math. Make sure you have the bike on a level surface. It is easier to get accurate measurements with two people. Once you have those measurements for the two bikes, you can see if you can get the same fit on the tandem as on your single by changing stem length and height.
My wife is 5'-8 1/2" and likes to ride pretty stretched out. She finds our Cannondale's stoker compartment too short. That doesn't prevent her from enjoying the ride, but it does limit the amount of time she can ride for. Thus why we just ordered a custom tandem with a 31" stoker compartment vs 28.5" on the Cannondale. We also know teams with stokers that are taller and perfectly comfortable with the same length stoker compartment.
Good luck on your tandeming journey!