Can you use a conventional water heater and a tank-less water heater at the same time?

I don't see why not; I've read that two conventional water heaters can be used at the same time to reduce electric costs, but I don't know how. You could hook them up in a series to pre-heat the water to say, 50 degrees, and hold it; then heat it to the desired temperature. Or you hook them up in parallel to heat different water systems at the same time, such as your kitchen and bathroom. I should also mention that I have no idea what I'm talking about.