Vxse / Vyse
It is certainly possible that you might be at the single engine service ceiling at take-off. The climb capability of the aircraft is dependent on a number of factors. However, I don't think having a common Vxse and Vyse speed are indicators of that condition.
I generally fly the Duchess fairly light, full fuel but only two people. The single engine rate of climb is downright respectable coming off the near sea level airports I usually fly out of and in to. I was two up and half fuel at 8000 last week doing a drag demo and we were getting climb rates in some configurations even up there. It was pretty cold.
The speeds are the same because when they tested the performance, those were the results. The only difference between a regular take-off and a short field take-off in the Duchess is full power before brake release and pull smartly, rather than gently, on the yoke at 71 KIAS while pitching for 85 KIAS.
CalifDan