My old range from the 1970s has two 30A circuits coming to it. New ranges all use a single 40A circuit. Can I join the two sets of wires into a single 60A breaker? The circuits would be joined at both ends to a new 60A wiring, one to range and one to new breaker.
Bottom line is NO, you definitely cannot do what you propose. You CANNOT parallel a circuit like this of this size. It is expressly forbidden in the NEC, and could very well be a safety hazard.
If the appliance takes two separate circuits like your old one that is a different story. Do you know what size and type of wire is feeding the old 30A circuits?
Also, all "new ranges" definitely do not use a 40A circuit. Many can, but many also have too great a draw for a 40A circuit.
The NEC allows any household cooking appliance 12kW or under to be served by a 40A circuit, but I see more and more basic units over 12kW.