Exmark is supposed to have 3 different hydro drive aerators. They still only currently have 2. The two available are the single hydro drive walk behind (21") and the ride-on. The third not yet available is the dual-hydro walk behind (24"). That is the one that you can put a sulky on. You cannot put a sulky on the other walk behind.
You will not find a demo for a dual hydro because they are not available yet. I would love to know why. I have been trying to buy one since they were still Lawn Solutions brand which was over a year ago. I waited until almost fall and with no availability date given from Exmark, I bit the bullet and bought the ride-on. Personally I have zero regrets about my purchase although it is a very expensive piece. But it completely removes the work out of aerating. It is actually easier to aerate than to mow a lawn! It is extremely productive. You can do several lawns big and small with no fatigue and no limitations on size of lawn. That is nice. Plus it pulls the best plugs that I have ever seen from any other aerator in my 16 years experience. I am not sure where you are getting quoted at 11k, but you should be able to get one for $8500 or less + tax (still not cheap, I know). In my opinion, worth every penny if you can get the work. I am happy now that I did not get the dual hydro. But, as someone else posted, you have to consider the size of machine. The ride on is 48" wide, so no small back gates. I also have the 21" single hydro version for that. Lucky for me, I put less than 2 hours on that this year. The rest was all ride-on.
I think if the dual-hydro was available and you could only have one aerator, it may be the one to own if fenced in back yards was a big concern for you. If your main concern is just plain productivity, the ride-on is the way to go.
As Rabbitman has posted, he has a Dual-Hydro for sale with the Lawn Solutions brand. Might be a way to get a deal! Not sure what he is asking or how many hours is on his machine. Is the sulky included with his?
Hope some of this info helps. You may also get some decent information from the thread "Your Machine Is In..." started by Rick13.