You do have to get a pole, to hitch a Pair to the surrey. A drop pole is the model you need, and a light neckyoke would go on the end to hold the pole up.
We call that model an Auto-Top surrey because the top folds down like old cars did. Never had fringe on it. Handy because the folding roof lets you put it inside shorter places than the solid roof type surrey does.
If you plan to hook the horses as a Pair, the harness will need a bit of modification from single horse use. You will also need a set of Pair reins. No, you can't modify the single horse reins to make it work. Light buggy collars, hame traces, are suggested for use with a pole, pads their necks better than breastcollar harness.