A tankless water heater(54amp) requires 2- 30 amp breakers. My old water heater has 1 -30 amp breaker on 10/2 wire. I want to know if I could change my 30amp dryer breaker to a 60 amp and install a sub panel on it with 2 - 30 amp breakers? 1 going to the heater and 1 for the dryer. I think the wire is 8 awg. I guess I could also do this on my range line, its already on a 50 amp breaker. My main panel is located so as I can not run any new wires from it.

I am not sure what the final result will be. The old 30 am water heater line, and the new 30 a line from the subpanel to provide the two 30 amp feeds to the new heater?

If that is true then it sounds OK, just the new subpanel will need a 60 amp feed with #6 copper wire, #8 will not be sufficient.

I really don't get these tankless electric units, the power they need to work is huge. I understand that there is no need to heat a tank of water and let it loose heat, but, what do I know. I guess the benefit is instant hot water rather than energy savings.

I have installed many, Don't think I want one. Even may be more energy efficient.
I would rather wrap tank with insulation.

This link has some info:

Top-Rated Water Heaters

Do you have 3/4" conduit going to panel?