They are in tandem because they power a 220 VAC circuit, rather then a 110 VAC circuit.
You might want to call an electrician... nothing usually happens when you touch two hots together.
I think you first need to figure out what size waterheater you need, then see what your amp rating is for that waterheater. Typicall Waterheater is usually rated for 30amps 240volts
10 awg wire can have a breaker size of 30amps. It can actually carry more but as for codes the wire is usually allowed to carry 80% of its max capability which puts the breaker at a max size of 30 amps.
30amps
On most residential circuit breaker boxes in North America, each individual breaker represents one 120-volt circuit. Two breakers ganged together represent a 240-volt circuit.
The electrical terminology for this type of breaker is a tandem breaker. It is where two breakers circuits can be in the same one breaker slot in a distribution panel. On this type of breaker there will be two output for a circuit value of what ever the breaker is rated for.
No, add new breaker,find a junction box and split the series, or add a box and split the load. You only need to do this if the breaker is tripping from overload. 12ga wire should have a 20amp breaker not a 15amp. If I understand your question,wired in parallel, this would be one hot connected to two breakers, first off two breakers is 220v not 120v , and 220v has two hot wires. Never connect two breakers together on one line.
30amps
Two single-pole 30 amp breakers tied together make a two-pole 30 amp breaker. From this you can power a device that requires 220-240 volts, typically.
One and half breaker system is an improvement on the double breaker system to effect saving in the number of circuit breakers. For every 2 circuits, 1 spare breaker is provided: Two feeders are fed from two buses via their associated circuit breakers and these two feeders are coupled by a third circuit breaker which is called tie breaker. During failure of any of the two feeder breakers, the power is fed via the breaker of the second feeder and main breaker (tie breaker).
I think you are asking how do you fix or replace one breaker on a double pole breaker? Most if not all double breakers have a connector on the switch so that they both trip at the same time. They are also joined together in the body of the breaker. You have to change both of them. If that's not your question, please explain further.
To do so LEGALLY, the circuit breaker must not be rated at a capacity (in AMPS) greater than the SMALLEST wire.