It sounds like you may be mistaken as to how this is wired, or that perhaps I'm just not understanding your explanation. As others have mentioned, it's not possible to get 240 volts from a single pole in a 120/240V split phase system. Each tandem breaker provides 2 120 V circuits, this is true. However, if you measure between the terminals on a single tandem breaker, you'll get 0 volts. This is because the terminals are both powered from the same leg, and so are at the same voltage potential. If you measure from a terminal on the top tandem breaker to a terminal on the bottom one, then you'll measure 240 volts. This is because each breaker is connected to a different leg, which are each one half of a 240 volt circuit.
With all that said. For this setup to work, one appliance would have to be connected to both breaker. Something like this...
Notice that each appliance circuit has one wire connected to each of the tandem breakers. In this situation, you'd need a device like Speedy Petey shows.
Which ties the breaker handles together, to provide common trip characteristics.
Notice how the inner handles are tied together, and that the outer handles are also tied to each other. This way if either trip (or are turned off by the user), the entire circuit is shut off.
If this is wired the way you've explained, where the dryer is connected to the top tandem and the heater is connected to the bottom. Then there's some magic going on in those breakers.
I'm not sure where you live but US code calls for a minimum of #8 if you're using copper or #6 if you're using aluminum. There would be no advantage in using a larger wire size in this case because at 30ft you will have negligible losses.
I don't know what your voltage is or how many phases you have but for an example of what your losses may be, I will assume a worst case scenario - 100V single phase. If you used the #6 aluminum wire, you would have a maximum of 1.72% voltage drop (loss). If you used the copper #8, you would have a maximum voltage drop of 1.66%. And just remember, when you are calculating losses (voltage drop), the higher the voltage, the better because higher voltages have less voltage drop.
Note: I use the Southwire Voltage Drop Calculator for most of my voltage drop calculations. http://www.southwire.com/support/voltage-drop-calculator.htm
Best Answer
For a typical 240V tank style water heater you will need a 30A/240V circuit. No neutral is required.
You CAN use the white wire as a hot going to the breaker for a straight 240V circuit such as this. You just need to mark it as a hot conductor with a permanent marker or such. You can use any typical hot color such as black, red or blue. You DO NOT need to use a red wire.
Now....you ask "Can I install the red line to a second hot breaker". This implies there is a red wire in the cable. If this is the case then yes, you must use the red and you can just cap off the white in the water heater since it is not used.
You CANNOT simply use a "second hot breaker". You MUST use a two-pole breaker for a 240V circuit as previously mentioned.