You'll need to do three things to make it work well and not put too much strain on the towing mower. First off is steering. Your "slave" mower may not want to track very well behind the towing mower, i.e. it may not want to turn when you do. I've never had to pull my JD around, but none of my other mowers would follow the tow vehicle without skidding the front tires or having someone on the towed mower to steer. Therefore you may need your tow bar to pivot at the towed mower's front axle with a drag link attached to one of the steering knuckles so the front tires will turn when you turn. Otherwise you'll need to fab up some mounts and install some large casters like what's on the front of a ZTR.
Second, you'll probably need to remove the rear axle or at least remove the internals to make the mower roll as freely as possible. If it has a manual transmission, you may only have to put it in neutral. However, if it has a hydro tranny, then you'll have to remove it. You'll need to measure the existing axle diameter. For the sake of this post, assume the axle has a 3/4" diameter. You can replace the rear axle with a piece of 3/4" rod, two 3/4" pillow blocks, four washers and two cotter pins.
Third, you'll need to remove as much dead weight from the towed mower as possible to make it easier to pull. If you have any incline at all, you'll be amazed how much of a difference 50 pounds can make.
Last winter I built an offset mower to use in my yard. I thought about going the same route that you are considering, but decided for me it was eaiser to build it using parts from the donor mower. I'll see if I can find those pics and I'll post them here, maybe it will give you some ideas....or maybe not.