Depending on how the pipes are layed out, it may be cheaper to put in a second WH rather than a tankless. Feeding warm/hot water into the tankless would decrease it's load and allow it to heat the water hotter, but at some point, you'd be restricted by volume...as the input to the tankless system gets colder, it starts to limit how warm the output is. Since you'd only get maybe 40-gallons out of the WH that was hot, you'd then quickly get down to your normal cold water inlet temp, and that's the thing that dictates the size of the tankless you need. The tankless would need some fairly expensive flue setup and you'd probably need to upgrade the gas line and maybe meter size. You might not need to do that with a second tank WH, depends. A tank has the luxury of taking its time to reheat, a tankless needs to be big enough for the maximum load, so the burner sizes are often quite different.
I think you'd be better off with a second conventional WH in series. It would have been better if you'd sized what you have to your needs, though.