A two-pole twin circuit breaker.

Here is a link that might be useful: A two-pole twin circuit breaker.

What sort of panel do you have? Yes this can be done in many panels that can accept tandem or skinny breakers. Some times all you need is a handle tie to take two poles off adjacent (not the same) tandem breaker. GE sells skinny two poles (you need to stick them such that they straddle the standard slot.

It may cost less to use thin breakers for existing circuits to free up space.

Make sure the panel listing allows the thin breakers, and then replace four full size breakers and you will have opened up two full size slots.

Thanks for the responses, brickeyee's answer makes the most sense for my application, I can squeeze 8 circuits into 4 full size spaces in the bottom of my panel.