Electrical – change 240 breaker to 120

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.

Since this would be a sub-panel install, ground and neutral are NOT connected - they are isolated. There is but one bond between them per code, and that one already exists in the system that the power is coming from.

I'd tend to agree with S.P. that your prior experience is either too long ago or too unrelated to what you are doing for it to seem a great idea for you to dive into this, unless you'd care to prove me (us) wrong by actually doing some research and modifying your question. Going into it half-cocked and not actually understanding things like why there are 4 wires and the neutral (grounded) and ground (grounding) conductors are isolated seems a recipe for disaster I'd rather not encourage.