I have an issue where I have 2-120V lines coming from a double pole 30. They both read 120 on my multimeter when I touch each line individually and connect the other side to a ground... however when I touch them together at the thermostat I don't get any reading at all. The resistance and everything on the heating elements are fine but somehow the power that's coming from the box is not combining to power the heater. Can anyone help

-Highly Frustrated

If the breaker was just installed, and happens to be a tandem breaker, it is installed so it is on the same leg or bus.

If this breaker has been in for a while, and did work, then sounds like one leg of the breaker is defective.

More detail would be helpful.

Any chance it's a Federal Pacific panel?

What happens when you measure them together at the breaker? Sounds like they're both on the same phase.