You may be right. I was just thinking each breaker could carry 30 amps each. Concrete, water, grass etc are all problematic. You are right. I was thinking at the panel.I don't think you "add them up". The total amperage draw is shown when measuring either of the two legs.
Also, why would concrete be any worse than standing on turf, for example?
Edit, in retrospect, I think you are correct and they are not added. Thanks