I bought a heater (ecosmart11), it needs 60amp. In the building I live I can't change the wiring from 40 to 60 (I bought the place, tho). So my 60amp breaker is useless. But, I can put two 30amp breakers. If I "split" the cable and instead of black, red and green, I have 6 cables, and plug in each group on each breaker, with it works?
You definitely do not want to do this. One of the reasons is if one of those 30A breakers were to trip it would leave the other half of the circuit to carry more current than it would normally be safe for it to handle.
Sure you might say that if the second breaker circuit saw a current overload then that breaker would also trip but this is not always assured. A 30A breaker is not going to instantly trip at say 30.001A. Whatever operating margin that second breaker would allow continued current to flow to the load overrating the 30A conductor on that half of the circuit.
So do this right and install a 60A breaker and new wiring suitable for 60A circuit.
The issue isn't the breaker - that's the easy part - it's the wire.
If the wire is only sufficient for 40A, there's no way around that. If you ran two 30A breakers in parallel on that wire, it would still be overloaded, and you'd have additional code compliance / safety issues detailed in another answer.
You can run heavier loads on a wire by increasing the voltage with transformers, but that is not likely practical in your situation.
Nope. The EcoSmart 11 goes back to the shop, and you get an EcoSmart 8 and a low-flow shower head.
Simple as that, if you are not willing to upgrade the wire. The root problem is the wiring in the wall is too small to be safe at 60A.
In addition to (and because of) all of those technical reasons, it is illegal to do what you propose.